345. Reverse Vowels of a String

O(n) time O(1) space

class Solution {
public:
    string reverseVowels(string s) {
        bool m[256] = {false};
        for (char v : "aeiouAEIOU") {
            m[v] = true;
        }
        int n = size(s), l = 0, r = n - 1;
        while (l < r) {
            while (l < r && !m[s[l]]) ++l;
            while (l < r && !m[s[r]]) --r;
            swap(s[l++], s[r--]);
        }
        return s;
    }
};