Software Engineering Interview Questions

What is the time complexity of accessing an element by index in an array?

Which operation has O(n) worst-case time when performed at the beginning of an array?

What is the space complexity of a prefix sum array for n elements?

What does nums.slice(1,3) return for nums = [10,20,30,40,50]?

Output of [1,2,3].map(x=>x*2).filter(x=>x>2) in JavaScript?

In a variable sliding window, what triggers moving the left pointer inward?

What is the time complexity of the sliding window solution to Longest Substring Without Repeating Characters?

Given nums = [-2,1,-3,4,-1,2,1,-5,4], what does Kadane's algorithm return?

Implement two-sum for an unsorted array returning indices. One solution guaranteed.

What is the amortized time complexity of Array.push() in JavaScript?

Why is string concatenation in a loop O(n²) in JavaScript?

What does Product of Array Except Self return for [1,2,3,4]?

What overlap condition must be true for intervals [a,b] and [c,d] (a<=c) to overlap?

After fixing nums[i] in 3Sum, what is the two-pointer time complexity for finding all pairs?

Time complexity of searching in an n×m matrix where rows and columns are sorted (LC 240)?

Why must you sort by start time before merging intervals?

Time complexity of Group Anagrams (LC 49) using sorted string as hash key?

Worst-case time complexity of naive string search?

Time complexity of finding longest common prefix among n strings each of length ≤m?

What is the optimal time complexity for solving the 'Arrays & Strings' classic problem?

Explain why initializing the prefix-sum map with {0:1} is necessary for 'Subarray Sum Equals K'.

After finding a valid window in Minimum Window Substring, why advance the left pointer?

Describe the two-step in-place algorithm to rotate a matrix 90 degrees clockwise.

What distinguishes the Maximum Product Subarray trick from Kadane's?

Which greedy choice minimizes intervals removed to make the rest non-overlapping (LC 435)?

How does length-prefix encoding in LC 271 handle strings containing the separator character?

What is the key insight enabling O(n) O(1)-space two-pointer solution to Trapping Rain Water?

Describe Boyer-Moore Majority Vote algorithm.

In Longest Repeating Character Replacement (LC 424), why keep maxFreq non-decreasing even when window shrinks?

Greedy approach for Jump Game II (LC 45) — minimum jumps to reach end?

How to find subarrays with sum divisible by k using prefix sums (LC 523)?

When would you choose a different approach for Arrays & Strings problems?

How does Z-algorithm help in string pattern matching?

Design an optimal solution for a real-world application of Arrays & Strings.

What two properties must a problem have to be solvable with dynamic programming?

What is the time complexity of Longest Common Subsequence for strings of length m and n?

What is the recurrence relation for Coin Change (minimum coins) DP?

Explain the difference between memoization (top-down) and tabulation (bottom-up) DP.

What is dp[i][j] in the Edit Distance recurrence?

What is the base case for Edit Distance when one string is empty?

What does dp[i] represent in Word Break (LC 139)?

What is Unique Paths (LC 62) — how many paths from top-left to bottom-right in m×n grid?

What is the time complexity of Word Break (LC 139) with the DP approach?

Why does the naive recursive Fibonacci have O(2^n) time complexity?

What is the minimum number of coins needed to make amount=11 with coins=[1,5,6]?

What is the optimal substructure property and give an example.

What is the optimal time complexity for solving the 'Dynamic Programming' classic problem?

House Robber circular (LC 213): why do two linear passes work?

In 0/1 Knapsack with space optimization, why must you iterate weight in DESCENDING order?

Coin Change II (LC 518) counts combinations, not permutations. How does loop order enforce this?

In the stock problem state machine (LC 309 with cooldown), what are the three states?

Time complexity of Burst Balloons (LC 312)?

How does Delete and Earn (LC 740) reduce to the House Robber problem?

Why is Decode Ways (LC 91) a DP problem and what are the two choices at each digit?

Partition Equal Subset Sum (LC 416): what DP state do you use?

What is the space-optimized form of LCS that uses O(n) space?

How does the sliding window (rolling array) optimization reduce Edit Distance space from O(m*n) to O(n)?

Palindromic Substrings (LC 647): what is the DP approach and what does dp[i][j] represent?

What is 'interval DP' and when do you use it?

What is the state for Burst Balloons (LC 312) interval DP?

What is the O(n log n) algorithm for Longest Increasing Subsequence?

In Longest Increasing Subsequence O(n log n), what does tails[i] represent?

Target Sum (LC 494): how does it transform into subset sum with count?

In the stock problem Best Time to Buy and Sell Stock III (LC 123), what is the state?

When would you choose a different approach for Dynamic Programming problems?

Design an optimal solution for a real-world application of Dynamic Programming.

What does inorder traversal of a BST produce?

What is the time complexity of search in a balanced BST?

What does the right side view of a binary tree (LC 199) return?

Why does comparing a node only to its direct parent fail when validating a BST?

In Lowest Common Ancestor of a Binary Tree (LC 236), when does the algorithm return the current root?

What is the time complexity of the O(n) balanced-tree check (LC 110) that returns -1 for unbalanced?

Implement maxDepth of a binary tree recursively.

What is the difference between tree height and tree depth?

What is the inorder successor of a node in a BST?

Time complexity of constructing a BST from a sorted array?

Why is iterative inorder traversal useful even though recursive is simpler?

Explain how to check if a binary tree is symmetric (LC 101).

What is the time complexity of LCA in a BST vs a general binary tree?

What is the time and space complexity of level-order traversal?

What is the space complexity of recursive DFS on a balanced binary tree?

What property makes AVL trees different from regular BSTs?

What is the difference between a complete binary tree and a perfect binary tree?

Solve Range Sum of BST (LC 938): count sum of values in range [low, high].

What is the time complexity of finding the kth smallest element in a BST?

What is the optimal time complexity for solving the 'Trees & BST' classic problem?

What are the three cases in BST node deletion?

How do you serialize a binary tree to uniquely reconstruct it?

How does the Diameter of Binary Tree (LC 543) differ from max depth?

Construct Binary Tree from Preorder and Inorder (LC 105): how do you find the left subtree size?

What is the time complexity of BST Iterator (LC 173) nextSmallest() and hasNext() operations?

In Binary Tree Maximum Path Sum (LC 124), what does the 'gain' function return vs what it records?

In Populating Next Right Pointers (LC 116), how do you connect nodes across different parent subtrees without extra space?

What is the key insight for Flatten Binary Tree to Linked List (LC 114) in O(1) space?

For Binary Tree Cameras (LC 968), what do the three states of each node represent?

How many unique BSTs can be formed with n distinct keys?

When would you choose a different approach for Trees & BST problems?

What is Morris traversal and what makes it special?

Design an optimal solution for a real-world application of Trees & BST.

What is the time complexity of BFS on a graph with V vertices and E edges?

What is the difference between a directed and undirected graph?

What property of a graph must hold for topological sort to be possible?

What is the in-degree of a node in a directed graph?

What does an empty result from Kahn's topological sort indicate?

Time complexity of Dijkstra's algorithm with a binary min-heap?

Implement BFS to find shortest path length between source and destination in an unweighted graph.

How does multi-source BFS differ from single-source BFS?

Number of Islands (LC 200): what is the time and space complexity of the BFS approach?

Why must BFS (not DFS) be used to guarantee shortest path in unweighted graphs?

What makes a graph bipartite?

What is the time complexity of Floyd-Warshall all-pairs shortest path?

What is the space complexity of storing a graph as an adjacency matrix vs adjacency list?

How do you generate all 8 neighboring cells for a position (r,c) in a grid?

What is the time complexity of topological sort using Kahn's algorithm?

Why is the adjacency list preferred over adjacency matrix for most interview problems?

What is the optimal time complexity for solving the 'Graphs' classic problem?

Why does Dijkstra's algorithm fail with negative edge weights?

What optimizations make Union-Find nearly O(1) per operation?

In cycle detection for a directed graph using DFS, what do the three colors (0,1,2) represent?

Explain how to detect a redundant connection in an undirected graph using Union-Find.

Clone Graph (LC 133): why use a visited map from original node to cloned node?

What is the key difference between using DFS vs Union-Find for connected components?

How does the 'stale entry' check work in Dijkstra's priority queue implementation?

What is a strongly connected component (SCC) in a directed graph?

What does path compression do to the amortized complexity of Union-Find?

Minimum Cost to Connect All Points (LC 1584): which algorithm and why?

What is the Alien Dictionary (LC 269) approach?

Describe how to find all eventually safe nodes (LC 802) using topological sort.

Cheapest Flights Within K Stops (LC 787): why NOT use standard Dijkstra?

Accounts Merge (LC 721): how do you apply Union-Find to emails?

When would you choose a different approach for Graphs problems?

Design an optimal solution for a real-world application of Graphs.

Time complexity of accessing the kth element of a singly linked list?

Time complexity of reversing a singly linked list?

Output of reversing [1,2,3,4,5]?

Time complexity of inserting a node immediately AFTER a given node (pointer already in hand)?

Space complexity of Floyd's cycle detection?

Why is a dummy head node useful when manipulating linked lists?

In Floyd's cycle detection, if slow moves 1 step and fast moves 2 steps, what does their meeting guarantee?

What three pointer variables are needed for iterative linked list reversal?

Implement iterative linked list reversal.

What does dummy.next return after Merge Two Sorted Lists?

Why does LRU Cache require a doubly linked list rather than singly?

Both naive (two-pass) and slow/fast (one-pass) approaches to find list middle are O(n). What is the practical advantage of slow/fast?

Time and space complexity of Odd Even Linked List (LC 328)?

Why must you save next=cur.next BEFORE setting cur.next=prev in iterative reversal?

Time complexity of deleting tail node in a singly linked list without a tail pointer?

What distinguishes sentinel nodes from regular nodes in a doubly linked list?

Correct order of three operations in Reorder List (LC 143)?

Time complexity of finding kth node from END of singly linked list in one pass?

What is the optimal time complexity for solving the 'Linked Lists' classic problem?

Time and space complexity of merging K sorted linked lists using a min-heap?

Space complexity of top-down recursive merge sort on a linked list?

How does the two-pointer technique find intersection of two linked lists (LC 160)?

In Reverse Nodes in k-Group (LC 25), how do you verify k nodes remain before reversing?

How to solve Add Two Numbers II (LC 445) without modifying input lists?

Why is merge sort preferred over quicksort for linked lists?

How does recursive reversal of a linked list work?

What happens during a cache HIT in LRU Cache (LC 146)?

What does Rotate List (LC 61) do and what is the efficient approach?

After detecting a cycle (slow and fast have met), why reset slow to head and advance both by 1 to find cycle entry?

What is the interleaving approach to Copy List with Random Pointer (LC 138)?

Describe approach to Flatten a Multilevel Doubly Linked List (LC 430).

When would you choose a different approach for Linked Lists problems?

Design an optimal solution for a real-world application of Linked Lists.

What does RADIO stand for in the system design interview framework?

In a 45-minute system design interview, how many minutes should you spend on the Infrastructure (architecture diagram) phase?

Which of the following is a NON-functional requirement?

What HTTP status code should a URL shortener return when redirecting a user from the short URL to the original URL?

What is the correct REST URL for retrieving all tweets posted by a specific user?

What is the polyglot persistence pattern?

Why should you always announce phase transitions during a system design interview?

You need to store 500 billion social graph edges with sub-millisecond multi-hop traversal queries. Which database do you choose?

What pagination type should you use for an infinite-scroll social feed and why?

A database primary server goes down. What is the standard recovery sequence?

What is the circuit breaker pattern used for?

When should you use gRPC instead of REST for service-to-service communication?

How do you handle a client submitting the same write request twice due to a network retry?

Which phase of RADIO is most commonly run short by candidates, and why does this hurt their score?

Why is it better to use cursor-based pagination than offset pagination for a social media timeline?

What is the correct order of concern when selecting a database in a system design interview?

A system has 100K read QPS and 5K write QPS on a single PostgreSQL instance. Which scaling action addresses the bottleneck?

What is the trade-off introduced by sharding a database by user_id?

A ride-sharing app needs to find all available drivers within 5 km of a rider's location in under 10ms. What is the most appropriate technology?

What is the Saga pattern used for in microservices?

What does a load balancer primarily do?

Which load balancing algorithm always routes the same client IP to the same server?

L7 load balancers can route based on HTTP headers and URL paths.

AWS ALB (Application Load Balancer) operates at which OSI layer?

Explain the difference between active and passive health checks in load balancing.

A server has 100 active connections and another has 5. Which algorithm would route the next request to the less loaded server?

True or False: AWS ALB supports routing to Lambda functions as targets.

You have 3 servers: Server A (4 CPU), Server B (8 CPU), Server C (16 CPU). Which LB algorithm and weights would you configure?

Configure an Nginx upstream block that implements least-connections load balancing with server2 marked as backup.

Why does IP hash load balancing create problems when servers are behind NAT?

What is connection draining (deregistration delay) and why is it critical for deployments?

Compare the trade-offs between sticky sessions via IP hash vs cookie-based sticky sessions.

Your service has 4 backend servers. Server 1 goes down. With modulo hashing (client_id % 4), how many clients are remapped? With consistent hashing?

Analyze why least-connections is better than round-robin for long-polling API endpoints.

A load balancer with round-robin sends requests to 3 servers. Server 2 is slow (1000ms response) while servers 1 and 3 respond in 10ms. What happens to overall throughput?

Explain the 'power of two choices' load balancing strategy and why it outperforms random selection.

Design a highly available load balancer setup that eliminates the LB itself as a single point of failure.

Evaluate when you would choose AWS NLB over ALB for a production service.

Design a health check system for a load balancer that minimizes both false positives (removing healthy servers) and false negatives (keeping sick servers).

Design the load balancing architecture for a ride-sharing app like Uber handling 1 million RPS globally.

Which caching pattern checks the cache first, then the database on miss, then populates the cache?

Write-through caching always writes to both cache and database simultaneously.

Which Redis data structure is best for implementing a real-time leaderboard?

What is a cache stampede?

Explain negative caching and when to use it.

Your Redis cache has 1GB memory limit. What eviction policy would you choose for a web session store where you want to keep recent sessions?

Design a cache key naming scheme for a multi-tenant SaaS application.

How does the Cache-Control header 's-maxage' differ from 'max-age'?

True or False: Redis is always faster than Memcached for caching.

What is the write-around caching pattern and when would you use it?

What is the difference between Redis Sentinel and Redis Cluster?

What is stale-while-revalidate and how does it improve user experience?

Compare LRU vs LFU eviction policies. When would LFU outperform LRU?

A popular article on your news site has 100,000 concurrent readers. Its cache entry expires. How do you prevent the cache stampede?

How would you implement cache warming after a Redis server restart?

A cache has 95% hit rate. A new feature doubles the number of cacheable objects. What happens to hit rate and how do you address it?

Design the caching strategy for a social media news feed where users see posts from accounts they follow.

Explain cache coherence problems in a multi-region deployment and how to address them.

How does Facebook's TAO system differ from a simple Redis cache?

Design a multi-layer cache for a global video streaming service.

What does ACID stand for?

Which type of index is best for a query with a range condition like WHERE age BETWEEN 20 AND 30?

The left-prefix rule for composite indexes means a composite index on (a, b, c) can serve a query filtering only on column b.

What is denormalization and when is it appropriate?

True or False: A foreign key constraint automatically creates an index on the referencing column.

What is a covering index and how does it improve query performance?

How would you model a many-to-many relationship between Products and Tags?

Design a database schema for a Twitter-like system. What tables and indexes would you create?

Explain the N+1 query problem and how to fix it.

What is an index-only scan and when does it occur?

What is optimistic locking and when does it fail?

What is connection pooling and how do you size a connection pool?

A query using WHERE email = ? is slow despite an index on email. What could cause this?

What is the difference between REPEATABLE READ and SERIALIZABLE isolation levels?

When would you choose Cassandra over PostgreSQL?

How do you handle storing hierarchical/tree data in a relational database?

Design an audit trail system that records all changes to a users table.

Explain the CAP theorem as it relates to database selection.

A PostgreSQL table has 10 million rows. A query SELECT * FROM orders WHERE status = 'pending' runs a sequential scan despite an index on status. Why?

You need to add a NOT NULL column to a 500-million-row table in production. How do you do it without downtime?

Design a database schema for a multi-tenant SaaS CRM application.

What is Conway's Law?

Microservices should always share a single database to maintain data consistency.

Which pattern allows migrating from monolith to microservices incrementally without big-bang rewrite?

What is a bounded context in Domain-Driven Design?

True or False: gRPC is always better than REST for microservice communication.

Design the service decomposition for a food delivery app like DoorDash.

What is the bulkhead pattern and how does it relate to microservices resilience?

Compare client-side vs server-side service discovery.

What problems does the microservices pattern introduce that a monolith doesn't have?

How does the API Gateway pattern relate to the BFF (Backend for Frontend) pattern?

What is the primary advantage of Microservices Architecture?

When should you NOT use Microservices Architecture?

What is the time complexity of the core operation in Microservices Architecture?

How does Microservices Architecture handle failures?

How do you handle data consistency across microservices that own separate databases?

What is the outbox pattern and why is it important?

What is a service mesh and when is it justified?

How would you implement distributed tracing across 5 microservices?

Design the microservices communication strategy for a real-time ride-matching service.

Design a microservices architecture that handles Black Friday traffic (100x normal load).

What is the time complexity of binary search on a sorted array of n elements?

Which sorting algorithm is stable and guarantees O(n log n) worst case?

What is the space complexity of mergesort?

Why does quicksort degrade to O(n²) on already-sorted input with last-element pivot?

Implement binary search to find the leftmost occurrence of a target.

What is the time complexity of the 'binary search on answer' approach for Koko Eating Bananas (LC 875)?

When should you use counting sort instead of comparison-based sort?

What is the loop termination condition for binary search on answer and why?

Find First and Last Position (LC 34): what two binary searches do you run?

What is Dutch National Flag problem (LC 75) and what algorithm solves it?

What is the time complexity of Median of Two Sorted Arrays (LC 4)?

Why is heapsort in-place while mergesort requires O(n) extra space?

Count inversions in [3,1,2,4]: how many inversions are there?

What is the worst-case time and space complexity of quicksort?

What does stability in a sorting algorithm mean and why does it matter?

What is the optimal time complexity for solving the 'Sorting & Searching' classic problem?

In the Lomuto partition scheme, what loop invariant is maintained?

What makes binary search on rotated sorted array (LC 33) more complex than standard binary search?

Quickselect finds the kth largest element in O(n) average. Why is it not O(n log n)?

Search a 2D Matrix (LC 74) in O(log(m*n)): how do you treat it as a sorted 1D array?

What is the lower bound on the number of comparisons needed to sort n elements?

Implement the Lomuto partition function for quicksort.

What is Timsort (used in Python and modern JS) and what makes it adaptive?

What happens at the boundary when lo==hi in binary search on answer?

For what type of problems is binary search on answer NOT applicable?

How does binary search apply to finding the minimum in a rotated sorted array (LC 153)?

How does mergesort count inversions (pairs where i<j but arr[i]>arr[j])?

Aggressive Cows (classic problem): how does binary search on answer apply?

Split Array Largest Sum (LC 410): what is the feasibility check for binary search on answer?

When would you choose a different approach for Sorting & Searching problems?

Design an optimal solution for a real-world application of Sorting & Searching.

What does LIFO stand for and which data structure follows this principle?

What is the time complexity of getMin() in a Min Stack using an auxiliary min-stack?

Why is array.shift() O(n) in JavaScript, and how can you avoid this for a queue?

In Valid Parentheses (LC 20), why pop and compare rather than just peek?

In Daily Temperatures (LC 739), why store indices rather than temperature values in the stack?

Time complexity of the monotonic stack approach to Next Greater Element?

What is the space complexity of BFS on a graph with V vertices and E edges?

Which expression evaluates correctly with RPN (Reverse Polish Notation): ['2','3','+','4','*']?

BFS vs DFS: which guarantees shortest path in an unweighted graph?

What is the space complexity of Largest Rectangle in Histogram (LC 84) using the monotonic stack?

What is the time complexity of Decode String (LC 394) and why?

Why does a stack naturally model DFS while a queue naturally models BFS?

Stack or Queue for implementing Undo/Redo in a text editor?

What does 'monotonic' mean in the context of a monotonic stack?

How many times is each element pushed and popped in the monotonic stack for Next Greater Element?

What is the worst-case time complexity of a single pop() operation in a two-stack queue?

What is the optimal time complexity for solving the 'Stacks & Queues' classic problem?

What is the amortized time complexity of dequeue in a two-stack queue?

What invariant does a monotonic decreasing stack maintain?

Implement a Min Stack with O(1) push, pop, top, and getMin.

Why must BFS mark nodes as visited BEFORE enqueuing rather than after dequeuing?

In Largest Rectangle in Histogram (LC 84), what does the sentinel 0 appended to heights array do?

What two stacks (or one stack of pairs) does Decode String (LC 394) use?

How does Rotting Oranges (LC 994) use multi-source BFS?

Car Fleet (LC 853): which pattern and why?

How do you represent state in Open the Lock (LC 752) BFS?

What is the time complexity of Word Ladder (LC 127) BFS?

What invariant does the deque maintain in Sliding Window Maximum (LC 239)?

What is the key insight for Basic Calculator (LC 224) handling parentheses?

Pacific Atlantic Water Flow (LC 417): why BFS from oceans rather than from cells?

Design a data structure supporting push, pop, top, and retrieveMax all in O(1).

Explain how to implement a stack using a single queue.

When would you choose a different approach for Stacks & Queues problems?

Design an optimal solution for a real-world application of Stacks & Queues.

What is the average time complexity of inserting into a hash table?

What is the time complexity of checking if a string is an anagram of another using a frequency array?

What is the difference between a JS Map and a plain Object for use as a hash map?

In Two Sum (LC 1), what is stored as key and value in the hash map?

Why does grouping anagrams with a sorted-string key cost O(n*k*log k) while a frequency-count key costs O(n*k)?

What does a hash collision mean and how does separate chaining resolve it?

What is the time complexity of Top K Frequent Elements (LC 347) with bucket sort?

What is the load factor of a hash table and why does it matter?

What are the two main collision resolution strategies?

What is the time complexity of Time Based Key-Value Store (LC 981) get() operation?

How do you use a hash set to detect if a linked list has a cycle?

What hash key would you use to group strings that are 'scramble anagrams' (same chars in same or different order)?

What is the space complexity of Group Anagrams (LC 49) with n strings of length k?

What is the average time to find all subarrays with sum equals k using a hash map?

Why does validating an anagram with a fixed-size array (size 26) use O(1) space?

How does Isomorphic Strings (LC 205) differ from checking if strings are anagrams?

In Happy Number (LC 202), why can you use a set instead of Floyd's cycle detection?

What is the optimal time complexity for solving the 'Hash Tables' classic problem?

In Subarray Sum Equals K (LC 560), why initialize the map with {0:1}?

Longest Consecutive Sequence (LC 128): why check only from sequence STARTS?

In Word Pattern (LC 290), why do you need TWO maps?

How does Contiguous Array (LC 525) convert the equal-zeros-and-ones problem to a prefix sum problem?

Four Sum Count (LC 454): how does splitting into two two-sum problems reduce O(n⁴) to O(n²)?

What is open addressing with linear probing?

Repeated DNA Sequences (LC 187): find all 10-char sequences that appear more than once.

How does Insert Delete GetRandom O(1) (LC 380) achieve O(1) deletion from the middle of the value set?

What trick does First Missing Positive (LC 41) use to achieve O(n) time and O(1) space?

What is a perfect hash function?

When would you choose a different approach for Hash Tables problems?

Explain how Rabin-Karp rolling hash enables O(n) substring search.

LFU Cache (LC 460): what is different from LRU and what data structures does it use?

Design an optimal solution for a real-world application of Hash Tables.

In a min-heap stored as an array, what is the parent index of the node at index i?

What is the time complexity of extracting the minimum from a min-heap?

What is the heap invariant for a min-heap?

What is the time complexity of peek() (see minimum without removing) in a min-heap?

Why use a min-heap of size k to find the kth LARGEST element?

What is the time complexity of building a heap from n elements (Floyd's heapify)?

What is the time complexity of merging K sorted lists of total N elements using a min-heap?

How do you simulate a max-heap using a min-heap in JavaScript?

What is the difference between a heap and a sorted array for priority queue operations?

In heapsort, after building the max-heap, what is the next step?

Why is the heap called a 'partially sorted' structure?

What is the time complexity of Reorganize String (LC 767)?

What is the time and space complexity of the two-heap MedianFinder?

What happens if you insert into a max-heap a value smaller than the root?

What is the optimal time complexity for solving the 'Heaps & Priority Queues' classic problem?

Explain the two-heap approach to Find Median from Data Stream.

Task Scheduler (LC 621): what determines the minimum intervals needed?

How does Connect Ropes for Minimum Cost work with a heap?

When would you prefer a sorted array over a heap as a priority queue?

Why is heapsort not stable?

What is the Kth Smallest Element in a Sorted Matrix (LC 378) approach using binary search vs heap?

Find K Pairs with Smallest Sums (LC 373): what initial entries go into the heap?

Furthest Building (LC 1642): what is the heap-based greedy strategy?

IPO (LC 502): why use two heaps — one for capital and one for profit?

Design Twitter (LC 355): how do you implement getNewsFeed efficiently with a heap?

Swim in Rising Water (LC 778): how is this a heap problem?

Explain why buildHeap (Floyd's algorithm) is O(n) rather than O(n log n).

When would you choose a different approach for Heaps & Priority Queues problems?

Sliding Window Median (LC 480): how do you handle element removal from the two heaps?

What is the advantage of a d-ary heap over a binary heap for Dijkstra's algorithm?

Design an optimal solution for a real-world application of Heaps & Priority Queues.

What is the recursion base case for Combination Sum (LC 39)?

What does the 'un-choose' step in backtracking accomplish?

For Letter Combinations of Phone Number (LC 17) with input '23', how many combinations are there?

What is the key difference between permutations and combinations in backtracking?

In backtracking, why must you push `[...path]` instead of `path` to the result?

Time complexity of generating all permutations of n distinct elements?

What is the choose/explore/un-choose pattern in backtracking?

In Combination Sum (LC 39), why sort the candidates array first?

Implement the backtracking solution to generate all subsets of [1,2,3].

In Word Search (LC 79), why mark the cell with '#' instead of using a separate visited set?

Generate Parentheses (LC 22): what are the two pruning conditions?

What is the time complexity of generating all combinations C(n,k)?

N-Queens: how many solutions exist for n=4?

How does the start index in the combinations template prevent duplicate combinations?

What is the relationship between backtracking and DFS?

What is the time complexity of Word Search (LC 79)?

What are the three types of conflict in N-Queens (same row, column, diagonal)?

What is the optimal time complexity for solving the 'Recursion & Backtracking' classic problem?

In N-Queens, why does row-col uniquely identify a diagonal?

What is the time complexity of Sudoku Solver in the worst case?

How do you handle duplicate elements in Subsets II (LC 90)?

In the Combination Sum II (LC 40), how do you skip duplicates at the same recursion level?

What is the branching factor and depth of the recursion tree for Permutations(n)?

Palindrome Partitioning (LC 131): how do you efficiently check if a substring is a palindrome?

In Restore IP Addresses (LC 93), what constraints limit each segment?

Why is backtracking considered 'exhaustive' yet often practical?

Beautiful Arrangement (LC 526): what constraint is checked at each position?

Why does Permutations II (LC 47) skip nums[i] when nums[i]==nums[i-1] AND !used[i-1]?

How does the Trie improve Word Search II (LC 212) compared to running LC 79 for each word?

When would you choose a different approach for Recursion & Backtracking problems?

Expression Add Operators (LC 282): why track the 'previous operand' in the backtracking state?

Design an optimal solution for a real-world application of Recursion & Backtracking.

What is the primary purpose of an API Gateway?

API Gateways can perform SSL termination, meaning they decrypt HTTPS traffic and forward HTTP to backends.

Which of the following is NOT typically handled by an API Gateway?

What is the circuit breaker pattern in the context of an API Gateway?

Explain the difference between an API Gateway and a Load Balancer.

What HTTP status code should an API gateway return when the circuit is open?

What is the BFF (Backend for Frontend) pattern?

You have a mobile app and a web app consuming the same API. Mobile needs compact JSON responses, web needs full responses. Which pattern do you use?

Design the authentication flow for an API gateway handling JWT tokens with 100ms latency budget.

True or False: AWS HTTP API is always the better choice over REST API in AWS API Gateway.

How does distributed rate limiting work across 10 API gateway instances?

Compare URL path versioning vs header versioning for APIs. Which would you recommend and why?

A payment service is experiencing 30% error rate. How should the API gateway circuit breaker respond?

How do you handle WebSocket connections through an API Gateway?

Explain idempotency keys and how the API gateway can help implement them.

What is request collapsing (or request coalescing) at the API gateway level?

Evaluate the trade-offs of putting an API gateway vs using a service mesh (Istio) for cross-cutting concerns.

A developer reports that 10% of API requests are getting 401 errors even with valid tokens. How do you debug this at the gateway?

How would you implement request signing (like AWS Signature V4) at the API gateway?

Design an API gateway that handles 1 million requests per second with 99.99% availability.

Design an API versioning strategy for a public API used by 1000+ third-party developers.

What is the primary purpose of a read replica in a database setup?

In master-slave replication, replicas can accept write operations.

Which sharding strategy ensures that adding a new shard requires moving minimal data?

What is replication lag and why does it matter?

What is the difference between vertical partitioning and horizontal partitioning (sharding)?

True or False: Adding more read replicas always improves write throughput.

What is the main problem with modulo-based hash sharding when you need to add a new shard?

Explain geographic sharding and how it helps with GDPR compliance.

How does PostgreSQL table partitioning differ from sharding?

Explain synchronous vs asynchronous replication. When would you use each?

What is CQRS and what problem does it solve?

What is the difference between a partition key and a sort key in DynamoDB?

A company has a users table with 200M rows. Queries by user_id are fast but queries by email are slow. What is the solution?

What are virtual nodes in consistent hashing and why are they needed?

What is a hot shard and how do you detect and fix it?

Why is a foreign key relationship across shards problematic?

Explain the CAP trade-off between PostgreSQL and Cassandra.

Design a sharding strategy for a social network where you need to efficiently fetch all posts by a user AND efficiently fetch a global feed of recent posts.

Design a strategy for migrating from 4 shards to 8 shards with zero downtime.

Design a strategy for handling the 'celebrity problem' in a sharded social network database.

Design the database scaling architecture for a ride-sharing app with 5M rides per day globally.

What is the primary advantage of using a message queue over direct synchronous API calls?

What is the difference between at-least-once and at-most-once delivery?

In Kafka, what determines which partition a message is sent to when using a key?

AWS SQS Standard Queue guarantees exactly-once delivery.

What is a Dead Letter Queue (DLQ)?

What is the visibility timeout in SQS and why is it important?

True or False: Kafka deletes messages after they are consumed by a consumer group.

Implement an idempotent Kafka consumer for processing payment events.

What is the difference between Kafka's consumer group and a traditional queue consumer?

A Kafka topic has 6 partitions. Consumer group A has 4 consumers. How are partitions distributed?

Compare Kafka and RabbitMQ. When would you choose each?

What is the saga pattern and how does it solve distributed transactions?

Explain event sourcing and its trade-offs.

A consumer is processing Kafka messages slowly, causing increasing lag. How do you address this?

Explain choreography vs orchestration in the saga pattern.

How does Kafka handle broker failures without losing messages?

How do you achieve message ordering across multiple Kafka partitions?

Design a real-time notification system for a food delivery app using message queues.

How would you implement exactly-once semantics in Kafka?

Design a system to process 10 million IoT sensor readings per second using message queues.

In the CAP theorem, what does 'P' (Partition Tolerance) mean?

Raft requires a majority (quorum) of nodes to elect a leader.

Which databases are examples of CP systems?

What is eventual consistency?

True or False: Vector clocks can detect concurrent events, while Lamport timestamps cannot.

A distributed database processes a write. How many nodes must acknowledge before the client gets a success response?

For a 5-node distributed database, what R and W values guarantee strong consistency?

What is the two-generals problem?

How does consistent hashing relate to distributed systems consensus?

What happens to a Raft cluster when the leader node crashes?

Explain the split-brain problem and how Raft prevents it.

What is the difference between Lamport timestamps and vector clocks?

Explain the PACELC theorem and give an example of a PA/EL system.

What is hinted handoff in Cassandra and how does it improve availability?

Compare Paxos and Raft consensus algorithms.

Explain anti-entropy and how it maintains consistency in eventually consistent systems.

Design a distributed lock service from scratch.

What is linearizability and how does it differ from sequential consistency?

How does Google's Spanner achieve global consistency across data centers?

Design a distributed ID generator that produces unique, roughly-sorted IDs.

What is the FLP impossibility result?

What is the primary advantage of horizontal scaling over vertical scaling?

Back-pressure allows a consumer to signal a producer to slow its output rate.

Which of the following makes a service stateless?

Explain the bulkhead pattern with a real-world analogy.

What is sticky session and why is it considered an anti-pattern?

True or False: Vertical scaling eliminates the need for stateless service design.

An application server uses in-memory sessions. It has 5 instances behind a load balancer. What problem occurs and how do you fix it?

What is the difference between performance and scalability?

How does queue-based load leveling prevent system overload?

What are the 12-Factor App principles and why do they enable scalability?

A service makes calls to 3 external APIs. One API becomes slow. How does bulkhead help?

Explain the X, Y, Z axes of the Scale Cube.

What is Amdahl's Law parallel limit for a system where 5% of work must be sequential?

What is the primary advantage of Scalability Patterns?

Design a system that scales from 100 users to 1 million users over 12 months.

What is Amdahl's Law and how does it limit horizontal scaling?

How do you implement graceful shutdown to support auto-scaling?

Design an auto-scaling strategy for a video transcoding service with highly variable load.

How do you scale a WebSocket service horizontally?

Design a system to handle Black Friday traffic (100x normal load) for an e-commerce site.

What HTTP status code should a URL shortener return for a permanent redirect?

With Base62 encoding using 6 characters, how many unique short URLs can be generated?

Which ID generation strategy avoids collisions entirely without database lookups?

A URL shortener has 100M URLs and uses MD5 hash truncated to 6 Base62 chars. What is the main risk?

Two users simultaneously request a custom alias 'mylink'. How do you handle this safely?

Your URL shortener's Redis cache suddenly goes down. What happens to the system?

Your URL shortener goes viral — a single short URL is clicked 1 million times per second. How do you handle this 'hot key' problem?

Why would you use Cassandra for analytics instead of PostgreSQL?

What caching eviction policy is most appropriate for URL shortener cache?

You need to add a feature: 'show all URLs created by user X'. How does this affect your database design?

What is the purpose of a 'tombstone' record when a short URL is deleted?

How would you design a multi-region active-active URL shortener with no single region as master?

What is the read/write ratio typically assumed for a URL shortener?

A customer complains that clicking a link shows 'URL not found' even though the link was created 1 hour ago. What could cause this?

Should you use 301 or 302 redirect for maximum analytics accuracy? What are the trade-offs?

Your URL shortener needs to handle 350,000 redirect requests/sec at peak. How many Redis nodes do you need, assuming each handles 100,000 ops/sec?

How would you implement link expiration at scale without scanning the entire database?

How would you prevent your URL shortener from being used to spread malicious links?

Which data store is most appropriate for storing the core short_code → long_url mapping at scale?

How would you migrate from auto-increment IDs to Snowflake IDs in a live production system with zero downtime?

What happens when the same long URL is shortened twice by two different users?

You want to guarantee that a short URL redirect returns in <10ms p99. Which architecture achieves this?

Why is WebSocket preferred over HTTP for a chat application?

Alice is connected to Chat Server 1. Bob is connected to Chat Server 2. Alice sends Bob a message. How is it delivered?

Why is Cassandra chosen for message storage over PostgreSQL?

A chat server crashes while 50,000 users are connected. What happens to their messages and connections?

What does 'delivered' status mean vs 'read' status in WhatsApp?

You have a group chat with 100,000 members (think a public community). Someone sends a message. How do you fan-out?

How do you implement 'message ordering' when messages arrive out of order due to network issues?

How many WebSocket servers are needed for 100 million concurrent connections, if each server handles 50,000 connections?

A user receives 1,000 notifications while their phone is off. When they turn it on, what should happen?

Explain how you would implement end-to-end encryption for group messages.

What is the purpose of a Hybrid Logical Clock (HLC) in a distributed chat system?

How do you handle typing indicators without overloading the server?

What is the role of Kafka in the chat message pipeline?

Design the unread message count feature. How do you efficiently show '5 unread' per conversation?

Your Kafka cluster goes down. What happens to message delivery?

What Cassandra partition key design supports efficient message retrieval for a conversation?

How would you design message search ('find all messages containing hello') at scale?

What is the difference between push notifications and WebSocket delivery? When do you use each?

What mechanism prevents a message from being delivered twice if the sender retries due to network timeout?

A chat server needs to be deployed with zero downtime. How do you drain WebSocket connections?

How do you handle the case where two users send messages to each other at the exact same millisecond?

What is the 'session affinity' or 'sticky session' problem with WebSocket servers?

What is 'fan-out on write' in the context of a news feed?

A celebrity has 50 million followers and posts 10 times per day. Fan-out on write generates how many feed updates per day?

Why is cursor-based pagination preferred over offset-based for news feeds?

A user follows both Taylor Swift (50M followers) and a friend (100 followers). How do you merge their posts into one ranked feed?

What data structure in Redis is best for storing a user's feed (ordered list of post IDs)?

Your Kafka fan-out workers fall behind during a viral event. Followers see the post 10 minutes late. How do you handle this?

How do you efficiently count likes on a post when 1 million people like it within 1 minute?

What is the 'thundering herd' problem in a news feed system?

How would you design the 'who liked this post' feature showing mutual friends first?

How do you prevent a user from seeing posts they've already seen when infinite scrolling loads the next page?

Which signal is most important for ranking a post higher in the news feed?

A user deactivates their account. How should this affect the news feed of their 10,000 followers?

What is the difference between a chronological feed and an algorithmic feed? What are the trade-offs?

How should media uploads be handled to avoid overloading the post service?

Your Redis cluster storing feed caches goes down. What is the fallback strategy?

How would you implement hashtag-based trending topics?

What approach does Facebook use to serve a single user's news feed from pre-computed data?

How do you ensure a user never sees a post they blocked or from a user they blocked?

How would you design A/B testing for news feed ranking algorithms?

What technique avoids the N+1 query problem when hydrating a list of 50 post IDs from cache?

A new user signs up and follows 0 people. What do you show in their empty feed?

How would you handle the news feed for a user who follows 10,000 accounts (power user)?

Which statement about let and var is TRUE?

What does console.log(typeof null) output?

What is the difference between == and ===?

Where does the prototype chain end?

What is the difference between forEach and map?

What is the difference between null and undefined in JavaScript?

What is a closure in JavaScript?

What is the Temporal Dead Zone (TDZ)?

What does Promise.all do if one promise rejects?

What output? ```javascript const obj = { x: 1 }; const copy = obj; copy.x = 2; console.log(obj.x); ```

What is hoisting and how does it differ for var, let, const, and function declarations?

What is the difference between call, apply, and bind?

What does Object.freeze do?

What does the async keyword do to a function's return value?

What is optional chaining (?.) and nullish coalescing (??)?

What does Array.prototype.reduce do? Implement sum.

Predict output: ```javascript const a = [1, 2, 3]; const b = [...a]; b.push(4); console.log(a.length, b.length); ```

What output does this produce? ```javascript for (var i = 0; i < 3; i++) { setTimeout(() => console.log(i), 0); } ```

Implement a memoize function that caches results by arguments.

What are the 4 rules of this binding in order of precedence?

What is the difference between __proto__ and prototype?

How do arrow functions differ from regular functions regarding this?

What is a generator function and when would you use one?

Explain event delegation and why it is useful.

What is a WeakMap and how does it differ from Map?

Implement a once function — callable only once, returning cached result thereafter.

Explain the JavaScript event loop and the difference between microtasks and macrotasks.

What does this output? ```javascript console.log('1'); setTimeout(() => console.log('2'), 0); Promise.resolve().then(() => console.log('3')); console.log('4'); ```

Implement debounce from scratch.

Explain synchronous vs asynchronous error handling.

Implement throttle from scratch.

What is the Proxy object used for?

Implement Promise.all from scratch.

Design a pub/sub EventEmitter in JavaScript.

What are the two rules of React hooks?

What does the dependency array in useEffect do?

What is the difference between useMemo and useCallback?

What is reconciliation in React?

What does React.memo do?

What is the difference between controlled and uncontrolled components?

What is the Context API and when would you use it?

What does useRef do and when would you use it?

What is prop drilling and how do you solve it?

What is the purpose of the key prop?

What happens when you return a function from useEffect?

What output does this produce on the first render? ```jsx function Component() { const [count, setCount] = useState(0); useEffect(() => { setCount(count + 1); }, []); return <div>{count}</div>; } ```

When should you use useReducer instead of useState?

What is a stale closure in React?

Why should you not use array index as a key in React lists?

What causes infinite re-render loops in React?

What is the difference between server components and client components in Next.js?

How does useCallback help performance?

What is an error boundary and why must it be a class component?

What does React.lazy and Suspense do?

What is forwardRef used for?

What is the difference between useState initializer and useState with a function argument?

Implement a useLocalStorage custom hook.

Implement a useDebounce hook.

What is the compound component pattern?

Explain React Fiber.

What is the useTransition hook used for?

How would you implement optimistic updates in React?

What is the difference between useLayoutEffect and useEffect?

Why does React StrictMode double-invoke effects in development?

Implement useFetch — a hook that fetches data from a URL.

What are React Server Components and what problem do they solve?

What does LEFT JOIN return when there is no matching row in the right table?

What does `NULL + 1` evaluate to in SQL?

True or False: A FULL OUTER JOIN returns all rows from both tables, including unmatched rows from each.

What is the difference between a primary key and a unique constraint?

What is the purpose of `COALESCE` in SQL?

What is the difference between WHERE and HAVING?

What is the difference between RANK() and DENSE_RANK()?

What are the four ACID properties?

What is the difference between TRUNCATE and DELETE?

What does `COUNT(col)` vs `COUNT(*)` count?

Write a query to find customers who have placed more than 3 orders.

What is a self-join and when would you use one?

What does the SQL `EXISTS` operator do?

What is SQL injection and how do you prevent it?

Arrange these SQL clauses in their logical execution order: HAVING, FROM, SELECT, WHERE, GROUP BY, ORDER BY

What is the difference between UNION and UNION ALL?

Write a SQL query to find the second-highest salary.

Explain what a composite index is and how column order matters.

What is a correlated subquery and why can it be slow?

What is an index-only scan?

What is the N+1 query problem in SQL?

What is a covering index?

What is a deadlock in a database and how is it resolved?

What does `EXPLAIN ANALYZE` show in PostgreSQL?

What is the difference between a clustered and non-clustered index?

What is a window function PARTITION BY and how does it differ from GROUP BY?

How would you find duplicate rows in a table?

What is `JSONB` in PostgreSQL and why is it preferred over `JSON`?

Write a query using a window function to get the top 3 salaries per department.

Write a recursive CTE to traverse an employee hierarchy and show all subordinates of a manager.

What are transaction isolation levels and what phenomena do they prevent?

What does the 'O' in SOLID stand for and how do you achieve it?

What is Dependency Injection and why does it make code more testable?

What is the Observer pattern and where does it appear in real systems?

What is the difference between the Adapter and Facade patterns?

True or False: The Singleton pattern is always an antipattern.

What is the Repository pattern and why is it useful?

What is a God Class and how do you fix it?

What is the Interface Segregation Principle?

Explain the Builder pattern and when to use it over a constructor with many parameters.

What is 'Feature Envy' code smell?

Arrange these in the order of abstraction from lowest to highest: Implementation, Interface, Concrete Class, Abstract Class

True or False: The Observer pattern and Publish/Subscribe (pub/sub) pattern are identical.

What pattern do middleware systems (Express, Django, Laravel) implement?

What is the Liskov Substitution Principle and give an example of a violation?

What is the difference between the Factory Method and Abstract Factory patterns?

What problem does the Decorator pattern solve that subclassing cannot?

Implement the Singleton pattern in TypeScript with lazy initialization.

What is the Strategy pattern and how does it support the Open/Closed Principle?

What is the Command pattern and how does it enable undo functionality?

What is the Composite pattern?

What is the Template Method pattern and where does it appear in frameworks?

What is the Chain of Responsibility pattern?

What is 'Shotgun Surgery' antipattern and which SOLID principle does it violate?

What is the difference between Composition and Aggregation in OOP?

What is 'Primitive Obsession' and how do you fix it?

What is the Null Object pattern?

What is the Single Responsibility Principle in practice — how do you identify when a class has too many responsibilities?

What is the difference between the Proxy and Decorator patterns?

What is the Flyweight pattern and when is it useful?

Design a logging system using the appropriate design pattern that supports multiple outputs (console, file, network) and log levels.

True or False: The heap and stack grow in opposite directions in a typical process address space.

What is the purpose of the PCB (Process Control Block)?

What is the difference between a process and a thread?

What are the four Coffman conditions required for deadlock?

What is virtual memory and why does every process need its own?

What is a race condition? Give a concrete example.

What is a zombie process?

Explain the Round Robin scheduling algorithm and its tradeoffs.

What is the difference between fork() and exec()?

What does an inode contain?

What is a context switch and what information is saved/restored?

What are Unix signals and give 3 examples?

What is the difference between a hard link and a symbolic (soft) link?

What is the difference between preemptive and cooperative (non-preemptive) scheduling?

What is an orphan process?

What is the /proc filesystem in Linux?

What is the difference between a mutex and a semaphore?

What is a page fault and what happens when one occurs?

What is thrashing in OS and what causes it?

What is a condition variable and why must the condition be checked in a while loop?

What is copy-on-write (COW) and how does it make fork() efficient?

What is a spinlock and when is it appropriate?

What is the purpose of the TLB (Translation Lookaside Buffer)?

What is memory fragmentation and what are its two types?

What is the difference between a major and minor page fault?

Arrange these CPU scheduling metrics from the perspective of an interactive user from most important to least: Throughput, Response time, Turnaround time, Waiting time

What does the kernel do during a system call?

What is a binary semaphore and how does it differ from a mutex?

Explain the concept of CPU affinity and when you would use it.

What are Linux namespaces and how do they enable containers?

What does HTTPS add over HTTP?

What is the difference between GET and POST in REST APIs?

What is the difference between authentication and authorization?

What HTTP status code should you return when a client sends a valid request but doesn't have permission?

What is the difference between TCP and UDP?

Describe the TCP 3-way handshake.

Explain the DNS resolution process step by step.

What is a certificate authority and why is it needed for TLS?

What is CORS and why does it exist?

What is the difference between a 301 and 302 redirect?

What is an idempotent HTTP method?

True or False: WebSockets use HTTP for the initial connection.

What are the three parts of a JWT?

What is a CDN and how does it reduce latency?

What is the purpose of the OPTIONS HTTP method?

What is DNS TTL and what are the tradeoffs of low vs high TTL?

What is the difference between 400, 401, 403, and 404 status codes?

Arrange these layers from closest to hardware (lowest) to closest to the user (highest): Transport, Application, Network, Physical

What is the difference between HTTP/1.1, HTTP/2, and HTTP/3?

What is head-of-line blocking?

What is the purpose of HTTP headers like Cache-Control, ETag, and Last-Modified?

What is perfect forward secrecy in TLS?

What is the difference between OAuth2 Authorization Code flow and Client Credentials flow?

What is SSL stripping and how does HSTS prevent it?

What is gRPC and when would you choose it over REST?

Explain how HTTP caching with Cache-Control: no-cache differs from Cache-Control: no-store.

What is Server-Sent Events (SSE) and when would you use it instead of WebSockets?

What is TCP's congestion control and why is it needed?

What is Rate Limiting and how would you implement it?

What happens when you type a URL into a browser and press Enter?

What is the output of: String a = "hello"; String b = "hello"; System.out.println(a == b);

Which collection provides O(1) average-case time for get(), put(), and containsKey()?

What happens when you call stream() operations without a terminal operation?

Which keyword ensures a field is NOT serialized when using Java object serialization?

What is the primary difference between synchronized methods and volatile fields?

What does the 'final' keyword mean when applied to a class, a method, and a variable respectively?

Which of these correctly implements a checked exception?

What is the output of: List<Integer> list = Arrays.asList(1, 2, 3); list.add(4);

In Java generics, what does 'List<? extends Number>' allow you to do with the list?

What is the role of Young Generation in JVM garbage collection?

What does the final keyword mean when applied to a method?

Which Java feature allows you to process collections with functional-style operations?

What is the difference between == and .equals() in Java?

What happens when you call HashMap.put() with an existing key?

What is the time complexity of ArrayList.add(0, element)?

What is the difference between checked and unchecked exceptions?

What is the output of: System.out.println(10 + 20 + "Hello" + 10 + 20)?

Which collection should you use for thread-safe key-value operations?

What is the purpose of the volatile keyword in Java?

What is the purpose of the transient keyword?

What is the time complexity of binary search?

What pattern would you use for 'find the longest substring without repeating characters'?

What pattern for 'next greater element' problems?

When should you use BFS over DFS?

What data structure for 'find median in a stream'?

What is the time complexity of Union-Find with path compression and union by rank?

What is the primary advantage of DSA Coding Patterns?

When should you NOT use DSA Coding Patterns?

What is the time complexity of the core operation in DSA Coding Patterns?

How does DSA Coding Patterns handle failures?

Which company popularized the use of DSA Coding Patterns?

In the Two Pointers pattern for Container With Most Water, why do you move the shorter pointer?

When does greedy NOT work?

What does a CDN cache HIT mean?

Which Cache-Control directive tells shared caches (CDN) to cache for 1 hour, but browser should revalidate?

Push CDN pre-loads content to edge nodes before any user requests it.

What is origin shield and why is it important for high-traffic sites?

What is content versioning and why is it better than CDN purging for static assets?

A new product is launching at midnight with 1M users expected. How do you use CDN to prepare?

How does the Vary header affect CDN caching?

Design a CDN caching strategy for an e-commerce product catalog with real-time pricing.

What is the difference between CDN-level DDoS protection and application-level rate limiting?

How do you cache authenticated API responses in a CDN?

What is a Surrogate Key (Cache Tag) and which CDNs support it?

True or False: Cloudflare Workers can modify the response from origin before sending to user.

Design a CDN architecture for a live video streaming platform.

How does Anycast routing help CDN route users to the nearest edge node?

What HTTP status code does a CDN return for a conditional request when content hasn't changed?

How does Netflix Open Connect CDN differ from public CDN providers?

What is a cache poisoning attack on a CDN and how do you prevent it?

Calculate the CDN cache hit ratio needed to save $1M/year if origin bandwidth costs $0.10/GB and CDN bandwidth costs $0.01/GB at 10PB/month.

True or False: Using a CDN always improves performance for all types of content.

What is the primary advantage of Content Delivery Networks (CDN)?

What HTTP status code should a rate limiter return when a limit is exceeded?

Which rate limiting algorithm allows a burst of requests followed by a sustained rate?

The leaky bucket algorithm smooths output regardless of input burst rate.

Explain the boundary burst problem with fixed window rate limiting.

Implement a token bucket rate limiter in Redis that allows 10 RPS with burst of 50.

How do you implement distributed rate limiting across 10 API server instances?

What is the sliding window counter approximation and what is its error bound?

Design rate limiting for a payment API that processes financial transactions.

What are the trade-offs between rate limiting at the CDN edge vs at the application?

True or False: Rate limiting and circuit breaking serve the same purpose.

A user hits the rate limit. What should the API response include?

How do you implement rate limiting that doesn't have Redis as a single point of failure?

Explain the token bucket implementation in AWS API Gateway.

Design a rate limiter for a SaaS API with 3 tiers: Free (100/day), Pro (10k/day), Enterprise (unlimited).

What is adaptive rate limiting and when would you use it?

How does rate limiting interact with retry logic in API clients?

What is the primary advantage of Rate Limiting & Throttling?

When should you NOT use Rate Limiting & Throttling?

What is the time complexity of the core operation in Rate Limiting & Throttling?

How does Rate Limiting & Throttling handle failures?

What is the difference between authentication and authorization?

A JWT access token should be stored in localStorage for a web application.

Which password hashing algorithm is recommended for storing user passwords?

Explain the OAuth 2.0 authorization code flow step by step.

How do you revoke a JWT token that hasn't expired yet?

Design an authentication system for 50 microservices.

What is PKCE in OAuth 2.0 and why is it required for public clients?

Compare RBAC and ABAC. When would you use each?

How does mTLS provide authentication for service-to-service communication?

What is zero-trust architecture and how does it differ from perimeter-based security?

True or False: OAuth 2.0 access tokens should be stored in browser cookies.

Design a multi-tenant authorization system for a SaaS application.

How do you implement single sign-on (SSO) for 5 web applications?

What is a confused deputy attack in authorization?

How do you securely implement 'remember me' / persistent login?

What is OpenID Connect (OIDC) and how does it extend OAuth 2.0?

Implement password reset flow securely.

How do you propagate user identity across microservices?

What is the primary advantage of Authentication & Authorization?

When should you NOT use Authentication & Authorization?

What are Google's Four Golden Signals?

What does SLO stand for and what is it used for?

Prometheus uses a push model to receive metrics from services.

Explain the difference between a trace, a span, and a trace ID.

What is the error budget for a service with a 99.9% SLO over a 30-day period?

Design an observability strategy for a microservices e-commerce platform with 30 services.

What is high cardinality in metrics and why is it a problem for Prometheus?

Compare Prometheus and Datadog for production monitoring.

How do you implement distributed tracing without modifying every service's code?

What is a Prometheus Counter vs Gauge vs Histogram?

True or False: SLA should be set equal to your internal SLO.

Design alerting for a payment API with 99.95% SLO using error budget burn rate.

What is MTTD and MTTR? How do you minimize each?

How do you correlate logs, metrics, and traces for a single request?

What is synthetic monitoring and how does it complement real-user monitoring?

What is log aggregation and why is it necessary in a microservices environment?

Design an on-call rotation and incident response process for a 6-engineer team.

What is the primary advantage of Monitoring & Observability?

When should you NOT use Monitoring & Observability?

What is the time complexity of the core operation in Monitoring & Observability?

What is the main advantage of HTTP/2 over HTTP/1.1?

Why does DNS primarily use UDP instead of TCP?

WebSocket connections work over standard HTTP ports (80/443).

Explain what happens at each step when you type 'https://google.com' in a browser.

What is head-of-line blocking and how does HTTP/2 address it?

When would you choose WebSocket over SSE for a real-time application?

Compare gRPC and REST for an internal microservice API. Which would you choose?

Explain TLS certificate chain validation.

How does QUIC (HTTP/3) improve on TCP+TLS?

True or False: HTTP/3 uses TCP as its transport protocol.

Design a protocol for a real-time multiplayer game.

What is the difference between a CNAME and an A record in DNS?

Explain how long polling works and what its limitations are.

How do you handle WebSocket connections across multiple server instances?

What is HPACK header compression in HTTP/2 and why is it important?

What is TCP congestion control and how does it affect application performance?

What is the primary advantage of Networking Fundamentals?

When should you NOT use Networking Fundamentals?

What is the time complexity of the core operation in Networking Fundamentals?

How does Networking Fundamentals handle failures?

What is the main difference between a data lake and a data warehouse?

Why is Parquet preferred over CSV for analytics workloads?

Apache Flink processes data as micro-batches, like Spark Streaming.

What is the difference between ETL and ELT?

Design a data pipeline to process 1 billion events per day from user activity on a mobile app.

What is a watermark in stream processing and why is it needed?

Explain the Lambda architecture and its main criticism.

What is Change Data Capture (CDC) and why is it used?

Design a real-time recommendation engine that updates recommendations as users browse.

What is data skew in Spark and how do you fix it?

True or False: Apache Spark can only process batch data, not real-time streams.

What is a star schema and how does it differ from OLTP schema design?

How do you ensure exactly-once processing in a stream processing pipeline?

What is Parquet and what are its key benefits for data analytics?

Design a system to track real-time inventory levels across 10,000 warehouses.

Explain the concept of event time vs processing time in stream processing.

What is dbt and how does it fit into the modern data stack?

What is the primary advantage of Data Storage & Processing?

When should you NOT use Data Storage & Processing?

What is the time complexity of the core operation in Data Storage & Processing?

Which HTTP status code indicates a rate limit has been exceeded?

What is the main disadvantage of a fixed window counter rate limiter?

Why must the sliding window check-and-increment be done atomically in Redis?

Your rate limiter Redis cluster goes down during peak traffic. What should happen?

Which algorithm allows bursting up to bucket capacity while maintaining an average rate?

How do you rate limit a distributed denial-of-service (DDoS) attack from 10,000 different IP addresses?

A sliding window log stores every request timestamp. For 100 req/min limit with 10M users, how much Redis memory is needed?

An API has a limit of 1000 requests/hour. A user sends 999 requests in the first minute. What should happen for the remaining 59 minutes?

How would you implement different rate limits for free vs paid users?

What Redis command makes a rate limit counter expire automatically after the window?

How do you rate limit an API endpoint that is called from a mobile app where users share the same NAT IP?

How would you design a rate limiter that limits by cost rather than by count (e.g., expensive ML inference costs 10 units, simple lookup costs 1)?

Which header tells a client when their rate limit window resets?

You need to rate limit across 5 Redis shards. A user's requests may hit different shards. How do you ensure consistent counting?

How does the leaky bucket algorithm differ from token bucket, and when would you choose it?

Where should a rate limiter be implemented in the request pipeline?

An attacker makes exactly 99 requests per minute (limit is 100) from 1000 different user accounts. How do you detect and block this?

What are the trade-offs between implementing rate limiting at the application level vs at a reverse proxy (like NGINX)?

What is the 'sliding window counter' algorithm's approximation error rate?

How do you test that your rate limiter is working correctly in production without impacting users?

Design a rate limiter for a GraphQL API where a single request can include multiple queries.

What Redis data structure is used for implementing sliding window log rate limiting?

Why is BFS (Breadth-First Search) preferred over DFS for web crawling?

What is the purpose of a Bloom filter in a web crawler?

A website generates infinite unique URLs like /products?page=1, /products?page=2 ... /products?page=1000000. How do you handle this?

What does robots.txt's 'Crawl-delay: 10' directive mean?