DSA Coding Patterns

Master the 20 most important coding interview patterns. Each pattern includes template code in Python and TypeScript, LeetCode problems, and interview strategies. Understanding Two Pointers The two-pointer technique uses two indices to traverse a data structure, reducing time complexity from O(n²) to O(n). It works best on sorted arrays or linked lists. Variants: Opposite ends: left starts at 0, right at end — move inward (Two Sum, Container With Most Water) Same direction: slow/fast pointers — detect cycles, find middle (Floyd's algorithm) Sliding window: left/right expand and contract — substring/subarray problems When to use two pointers: "Find a pair that sums to X" → Opposite ends on sorted array "Remove duplicates in-place" → Slow (write) and fast (read) pointers "Is palindrome?" → Compare from both ends "Merge two sorted arrays" → One pointer per array Two Pointers Pattern The Two Pointers technique uses two references to traverse a data structure, often from opposite ends or at different speeds. When to Recognize Signal words: "sorted array", "pair that sums to", "remove duplicates", "palindrome" The input is sorted or can be sorted You need to find pairs or compare elements Template: Opposite Direction Template: Same Direction (Fast/Slow) LeetCode Problems #1 Two Sum (Easy) — hash map approach, but sorted variant uses two pointers #15 3Sum (Medium) — fix one element, two-pointer for remaining #11 Container With Most Water (Medium) — move the shorter side inward #26 Remove Duplicates from Sorted Array (Easy) #125 Valid Palindrome (Easy) Walkthrough: Container With Most Water (#11) Always move the pointer with the SHORTER height — moving the taller one can never increase the area. Time/Space: O(n) time, O(1) space Common Mistakes Forgetting the array must be sorted for the opposite-direction approach Off-by-one errors with vs Not handling duplicates (3Sum: skip duplicate values after finding a match) Interview Tip "When I see a sorted array with a pair-finding problem, my first instinct is two pointers from both ends. It gives O(n) time with O(1) space, which is optimal." Java Implementation