class Solution
{
public:
  //Insert one char from stringstream
    void Insert(char ch)
    {
         if(cimap.count(ch) == 0)
             cque.push(ch);
         ++cimap[ch];
    }
  //return the first appearence once char in current stringstream
    char FirstAppearingOnce()
    {
        while(!cque.empty()){
            if (cimap[cque.front()] > 1)
                cque.pop();
            else
                return cque.front();
        }
        return '#';
    }

    map<char, int> cimap;
    queue<char> cque;
};

用一个map和queue数据结构
1、map记录输入字符个数,queue无重复记录各字符输入顺序
2、输出时由队首开始检查输入个数,然后输出
欢迎交流指正!!!