Two Pointers & Sliding Window
Master 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.
Two Sum II
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)Valid Palindrome
Verify a string reads the same forwards and backwards by comparing characters from both ends, skipping non-alphanumeric characters.
O(n)O(1)Maximum Sum Subarray
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)Longest Substring Without Repeating Characters
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)Why Learn Two Pointers & Sliding Window?
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.
Linear Time Solutions
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.
Interview Staples
Two pointers and sliding window appear in dozens of popular LeetCode problems. Recognizing the pattern instantly gives you a clear solution path under pressure.
Real-world Applications
Network packet buffering, stream processing, substring matching in text editors, and DNA sequence analysis all rely on sliding window techniques at their core.