class Solution{
public:
    //Insert one char from stringstream
    unordered_map<char,int>map;
    queue<char> q;
    void insert(char ch){
       if(map[ch]==0) map[ch]=1,q.push(ch);
        else map[ch]++;
    }
    //return the first appearence once char in current stringstream
    char firstAppearingOnce(){
      while(q.size()&&map[q.front()]>1)
        q.pop();
        if(q.empty()) return'#';
        return q.front();
    }
};