请实现一个函数用来找出字符流中第一个只出现一次的字符

时间O(N) 空间O(n)

class Solution
{
public:
  //Insert one char from stringstream
    void Insert(char ch)
    {
        str += ch;
        mymap[ch]++;
    }
  //return the first appearence once char in current stringstream
    char FirstAppearingOnce()
    {    
        for(auto x:str){
            if(mymap[x] == 1)
                return x;
        }
        return '#';
    }

    private:
    unordered_map<char,int> mymap;
    string str;
};