Given a string s, reverse only the vowels in it and return the result — every consonant stays exactly where it is. Collecting every vowel into a separate list and handing them back out in reverse order works, but it costs an extra list the size of the vowel count. The two-pointerTwo PointerUsing two indices that move through one or more sequences to avoid redundant re-scanning. technique swaps vowels directly in place: walk pointers inward from both ends, sliding each one forward or backward until it lands on a vowel, then swap the pair and keep going. No extra storage is needed — the vowels trade positions as the pointers move toward the middle.