class Solution{
public:
    map<char, int> cnt;
    queue<char> chs;
    //Insert one char from stringstream
    void insert(char ch){
        if (++cnt[ch] > 1) while (!chs.empty() && cnt[chs.front()] > 1) chs.pop();
        else chs.push(ch);
    }
    //return the first appearence once char in current stringstream
    char firstAppearingOnce(){
        if (chs.empty()) return '#';
        else return chs.front();
    }
};
//grygTgXnfNfwxFOfHVlRDGtIrnTpBVpVzbeJsuJMBOQzMetlPdFOuWhZoWjZUolrlibIuSWKzkjHUQhvWqztHXpAAiAeYnbFPvog