Map<Character,Integer> map = new HashMap<Character, Integer>();
    ArrayList<Character> arrayList = new ArrayList<Character>();
    //Insert one char from stringstream
    public void Insert(char ch)
    {
        if (map.containsKey(ch)){
            map.put(ch,map.get(ch)
 
+1);
        }else {
            map.put(ch,1);
        }
        arrayList.add(ch);

    }
    //return the first appearence once char in current stringstream
    public  char FirstAppearingOnce()
    {
        char ch=‘#‘;
        for (int i=0;i<arrayList.size();i++){
            if (map.get(arrayList.get(i))==1){
                
 
return arrayList.get(i);
            }
        }
        return ch;
    }

牛客（54）字符流中第一個不重復的字符