Two-Pointer Problem

“Two-Pointer Problem” is not a full contract, so ask whether it is a sorted-array pair search, a sliding window, a partition, or a linked-list gap problem. For the standard sorted pair search, left and right bound the candidates still possible. When the current sum is too small, advance left; when too large, decrement right; equality yields the requested pair or count. The proof is that sorted order eliminates all pairs using the discarded endpoint in the wrong direction. This is O(n) time and O(1) extra space after sorting, or O(n log n) including sorting. For windows, the invariant instead concerns a feasible interval and both pointers only move forward. State the exact variant before implementation. Test empty input, a single value, duplicates, absent answers, negative values, and index requirements.