O(1)解法简版

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

};