Given a string s, return it reversed. The naive approach rebuilds the result one character at a time through string concatenation — correct, but wasteful in languages where strings are immutable, since every append can re-copy everything built so far. The two pointerTwo PointerUsing two indices that move toward (or away from) each other through a structure, instead of a single pass that only moves in one direction. technique avoids that entirely: convert the string to a character array and swap from both ends inward, using no more than a single temporary variable at any moment.