https://leetcode-cn.com/problems/implement-stack-using-queues/

    1. class MyStack {
    3.     Queue<Integer> queue;
    4.     public MyStack() {
    5.         queue  = new LinkedList<>();
    6.     }
    8.     public void push(int x) {
    9.         // 1. 首先记录当前队列长度
    10.         int size = queue.size();
    11.         // 2. 把 x 入队
    12.         queue.offer(x);
    13.         // 3. Q 中原先所有的元素先出队，然后再入队
    14.         for (int i = 0; i < size; i++) {
    15.             queue.offer(queue.poll());
    16.         }
    17.     }
    19.     public int pop() {
    20.         return queue.poll();
    21.     }
    23.     public int top() {
    24.         return queue.peek();
    25.     }
    27.     public boolean empty() {
    28.         return queue.isEmpty();
    29.     }
    30. }
    32. /**
    33.  * Your MyStack object will be instantiated and called as such:
    34.  * MyStack obj = new MyStack();
    35.  * obj.push(x);
    36.  * int param_2 = obj.pop();
    37.  * int param_3 = obj.top();
    38.  * boolean param_4 = obj.empty();
    39.  */