class Solution {

    Map<Character,Integer> map=new LinkedHashMap();

    //Insert one char from stringstream   
    public void insert(char ch){
        map.put(ch,map.getOrDefault(ch,0)+1);
    }
    //return the first appearence once char in current stringstream
    public char firstAppearingOnce(){
        for(Map.Entry<Character,Integer> entry:map.entrySet()){
            if(entry.getValue()==1){
                return entry.getKey();
            }
        }
        return '#';
    }
}