// Run-time complexity O(n2)
    // Memory complexity O(1)
    /*
    int firstUniqChar(string s) {
        for (int i = 0; i < s.size(); ++i) {
            if (count(s.begin(), s.end(), s[i]) == 1)
                return i;
        }
        return -1;
    }
    */

    // Run-time complexity O(n)
    // Memory complexity O(1)
    // 26 different letters
    int firstUniqChar(string s) {
        vector<int> abc(26, 0);

        for (char c : s) {
            abc[c - 'a'] += 1;
        }

        for (int i = 0; i < s.size(); ++i) {
            if (abc[s[i] - 'a'] == 1)
                return i;
        }
        return -1;
    }