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Two Pointers & Sliding Window
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Two Sum Pattern
2 + 6 = 82
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L=0target: 10R=9
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Two Pointers & Sliding Window
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Two Sum Pattern
2 + 6 = 8Master linear-time array and string techniques. Two pointers converge from both ends; sliding windows expand and contract to maintain invariants — both avoid nested loops entirely.
Classic two-pointer technique on a sorted array. Place one pointer at each end and converge inward, eliminating half the search space at every step.
O(n)O(1)Verify a string reads the same forwards and backwards by comparing characters from both ends, skipping non-alphanumeric characters.
O(n)O(1)Find the contiguous subarray of length k with the largest sum using a fixed-size sliding window that shifts one position at a time.
O(n)O(1)Expand the window rightward adding new characters; shrink from the left whenever a duplicate is detected. The window always holds a valid unique-character substring.
O(n)O(k)Given bar heights, find two bars that trap the most water. Two pointers converge inward, always moving the shorter bar — watch the water area update in real time.
O(n)O(1)Sort the array, fix a pivot, then run a two-pointer sweep on the remainder. Duplicate pivots and duplicate pairs are skipped to produce a unique set of triplets.
O(n²)O(1)Expand the right pointer until the window contains all required characters, then shrink from the left to find the tightest valid window. Track character frequencies to detect validity in O(1).
O(n + m)O(m)These patterns transform brute-force O(n²) solutions into elegant O(n) ones. They appear constantly in interviews and are foundational for array and string problems.
Replace nested loops with smart pointer movement. Both patterns achieve O(n) by ensuring each element is visited at most twice — once when added, once when removed.
Two pointers and sliding window appear in dozens of popular LeetCode problems. Recognizing the pattern instantly gives you a clear solution path under pressure.
Network packet buffering, stream processing, substring matching in text editors, and DNA sequence analysis all rely on sliding window techniques at their core.