s = "leetcode"
返回 0.
​
s = "loveleetcode",
返回 2.

class Solution {
public:
    int firstUniqChar(string s) {
        unordered_map<char,int> tmp;
        for(auto c: s)
            tmp[c]++;
        for(int i=0;i<s.length();i++)
        {
            if(tmp[s[i]]==1) return i;
        }
        return -1;
​
    }
};