剑指54-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)
    {
         m_str.push_back(ch);
    }
  //return the first appearence once char in current stringstream
    char FirstAppearingOnce()
    {
        map<char, int>str_map;
        for(auto item:m_str)
        {
            str_map[item]++;
        }
        for(auto item:m_str)
        {
            if(str_map[item]==1) return item;
        }
        return '#';
    }
private:
    string m_str;


};