class Solution{
public:
    unordered_map<char,int> count;
    queue<int> q;
    //Insert one char from stringstream
    void insert(char ch){
        if(++ count[ch] > 1) //当ch出现过时
        {
            while(q.size() && count[q.front()] > 1) q.pop(); //弹出队列中所有出现过不止一次的字母
        }
        else
        q.push(ch); //加入没有出现过的字母
    }
    //return the first appearence once char in current stringstream
    char firstAppearingOnce(){
        if(q.empty()) return '#';
        return q.front();
    }
};