leetcode---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”.

Note:
The vowels does not include the letter “y”.

class Solution {
public:
    string reverseVowels(string s) 
    {
        set<char> s1;
        s1.insert('a');
        s1.insert('e');
        s1.insert('i');
        s1.insert('o');
        s1.insert('u');
        s1.insert('A');
        s1.insert('E');
        s1.insert('I');
        s1.insert('O');
        s1.insert('U');

        stack<char> s2;
        vector<int> id;
        for(int i=0; i<s.size(); i++)
        {
            if(s1.count(s[i]))
            {
                s2.push(s[i]);
                id.push_back(i);
            }
        }

        for(int i=0; i<id.size(); i++)
        {
            s[id[i]] = s2.top();
            s2.pop();
        }

        return s;
    }
};