第一个只出现一次的字符

class Solution {
public:
    int FirstNotRepeatingChar(string str) {
        if (str.empty()) return -1;
        map<char, int> mp;
        for (int i = 0; i < str.size(); ++i)
            mp[str[i]]++;
        for (int i = 0; i < str.size(); ++i) {
            if (mp[str[i]] == 1)
                return i;
        }
        return -1;
    }
};