Approach: Two pointers

def reverseVowels(s):
    v = set("aeiouAEIOU")
    a = list(s); l, r = 0, len(a)-1
    while l < r:
        if a[l] not in v: l += 1
        elif a[r] not in v: r -= 1
        else: a[l], a[r] = a[r], a[l]; l += 1; r -= 1
    return "".join(a)

Complexity: O(n) time, O(n) space.

Full Solution — all languages

class Solution:
    def reverseVowels(self, s):
        v = set("aeiouAEIOU")
        a = list(s)
        l, r = 0, len(a) - 1
        while l < r:
            if a[l] not in v:
                l += 1
            elif a[r] not in v:
                r -= 1
            else:
                a[l], a[r] = a[r], a[l]
                l += 1
                r -= 1
        return "".join(a)

import java.util.*;
class Solution {
    public String reverseVowels(String s) {
        String v = "aeiouAEIOU";
        char[] a = s.toCharArray();
        int l = 0, r = a.length - 1;
        while (l < r) {
            if (v.indexOf(a[l]) < 0) l++;
            else if (v.indexOf(a[r]) < 0) r--;
            else {
                char t = a[l];
                a[l] = a[r];
                a[r] = t;
                l++;
                r--;
            }
        }
        return new String(a);
    }
}

#include <bits/stdc++.h> using namespace std;
class Solution {
    public: string reverseVowels(string s) {
        string v = "aeiouAEIOU";
        int l = 0, r = (int)s.size() - 1;
        while (l < r) {
            if (v.find(s[l]) == string::npos) l++;
            else if (v.find(s[r]) == string::npos) r--;
            else {
                swap(s[l], s[r]);
                l++;
                r--;
            }
        }
        return s;
    }
};

/**
 * @param {string} s
 * @return {string}
 */ var reverseVowels = function(s) {
    const v = "aeiouAEIOU";
    const a = s.split('');
    let l = 0, r = a.length - 1;
    while (l < r) {
        if (v.indexOf(a[l]) < 0) l++;
        else if (v.indexOf(a[r]) < 0) r--;
        else {
            const t = a[l];
            a[l] = a[r];
            a[r] = t;
            l++;
            r--;
        }
    }
    return a.join('');
};

Reverse Vowels of a String

Example 1

Example 2

Constraints