字符流中第一个不重复的字符

class Solution
{
public:
  //Insert one char from stringstream
    void Insert(char ch)
    {
        s+=ch;
        m[ch]++;
    }
  //return the first appearence once char in current stringstream
    char FirstAppearingOnce()
    {
        for(int i=0;i<s.size();i++){
            if(m[s[i]]==1) return s[i];
        }
        return '#';
    }
private:
    string s;
    unordered_map<char,int> m;
};