只用一个 LinkedList

题目抽象：每个字符依次进入队列，保证这个队列里面没有重复的字符，返回队首。

    LinkedList<Character> queue = new LinkedList<Character>();
    public void Insert(Character ch){
        if(queue.contains(ch)) {
            queue.remove(ch);
        }else {
            queue.offer(ch);
        }
    }
  //return the first appearence once char in current stringstream
    public char FirstAppearingOnce(){
        return queue.isEmpty() ? '#' : queue.peek();
    }