Reverse Vowels of a String
Easy · Two Pointers · Two Pointers · String
Problem
Given a string s, reverse the order of only its vowels and return the result. Every other character stays in its original position.
The vowels are a, e, i, o and u, in either lowercase or uppercase. A vowel keeps its own case when it moves.
Examples
Input: s = "programming" Output: "prigrammong"
The vowels o, a, i are written back as i, a, o; every consonant stays where it was.
Input: s = "EchO" Output: "OchE"
E and O swap, and each keeps its own case.
Input: s = "rhythm" Output: "rhythm"
There are no vowels (y is not one), so nothing moves.
Constraints
- 1 <= len(s) <= 3 · 10^5
- s consists of printable ASCII characters
Approaches
- Rebuild the string at every swap — time O(n · v), space O(n)
Follow-up
What if the string were a stream too long to hold, read once from each end? The same two pointers work on any sequence with random access from both ends, such as a file you can seek in: read a vowel from the front, find one from the back, and write both swapped. Without access from the back you would have to buffer the vowels, which is the stack approach.
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