int[] table=new int[256];
    StringBuffer sb=new StringBuffer(); 
    public void Insert(char ch)
    {
        sb.append(ch);
        table[ch]++;
    }
  //return the first appearence once char in current stringstream
    public char FirstAppearingOnce()
    {
        char[] str=sb.toString().toCharArray();
        for(char c:str){
            if(table[c]==1)
                return c;
        }
        return ‘#‘;
    }