Two Pointers Interview Questions & Solutions

The two-pointer technique is the elegant answer to problems where a naive solution uses a nested loop. By moving two indices through the data intelligently — from opposite ends, or at different speeds — you often replace an O(n²) scan with a single O(n) pass and no extra memory. Interviewers love these problems precisely because the brute force is obvious and the optimization is a genuine insight.

These solutions cover the main variations: the opposite-ends approach for sorted two-sum, container-with-most-water, and valid-palindrome checks; the fast-and-slow approach for in-place partitioning and removing elements; and the "advance the pointer that can't hurt you" reasoning that makes these solutions provably correct. A recurring theme is why the technique works on sorted data — understanding that is what lets you decide whether a quick sort up front is worth it. Master two pointers and sliding window (its close cousin) and you'll have clean, linear answers to a big slice of array and string interviews.