Two pointer

Two pointers is a technique, not a solution by itself. Ask whether the array is sorted, whether the goal is a pair, an in-place partition, a window, or linked-list cycle detection. For a sorted pair-sum target, hold left at the smallest and right at the largest candidate. If their sum is too small, increment left because no smaller-left partner can help; if too large, decrement right. That monotonic argument is the invariant that makes progress safe. Each pointer moves at most n times, yielding O(n) time and O(1) extra space after sorting; sorting an unsorted input costs O(n log n) and may lose original indices. Test duplicates, no answer, negative numbers, and left crossing right. Never move both pointers without explaining why candidates are eliminated.