Two-Pointer approach

Before using a two-pointer approach, name the ordering or monotonicity that supports it. In the common sorted-array pair problem, pointers delimit all remaining candidate pairs. Compare their values: a sum below target means moving the left pointer is safe because every pair with that left value and a smaller right value is no better; a sum above target means moving right is safe. This turns a quadratic nested loop into one linear sweep, O(n) time and O(1) auxiliary space after sorting. Other uses have different invariants: fast/slow pointers preserve a distance gap, and partition pointers preserve regions of classified elements. Ask which version the interviewer means. Test crossing pointers, duplicates, empty input, and overflow in arithmetic. A correct explanation proves why each move discards impossible candidates rather than merely describing pointer updates.