leetcode 345. Reverse Vowels of a String

Write a function that takes a string as input and reverse only the vowels of a string.
Example 1:
Given s = “hello”, return “holle”.
Example 2:
Given s = “leetcode”, return “leotcede”.
class Solution {
public:
unordered_set vowels = {‘a’, ‘e’, ‘i’, ‘o’, ‘u’, ‘A’, ‘E’, ‘I’, ‘O’, ‘U’};
string reverseVowels(string s) {

        int start = 0;
        int end = s.length() - 1;
        while(start < end) {
            if(vowels.find(s[start]) != vowels.end() && vowels.find(s[end]) != vowels.end()) {
                swap(s[start], s[end]);
                start++;
                end--;
            }
            else if(vowels.find(s[start]) != vowels.end()) {
                end--;
            }
            else if(vowels.find(s[end]) != vowels.end()) {
                start++;
            }
            else {
                start++;
                end--;
            }
        }
        return s;
    }
};