题目：

Implement the following operations of a queue using stacks.

使用堆栈实现队列的以下操作。

push(x) -- Push element x to the back of queue. 将元素x推送到队列的后面。

pop() -- Removes the element from in front of queue.从队列前面删除元素。

peek() -- Get the front element.获取前面的元素。

empty() -- Return whether the queue is empty. 返回队列是否为空。

Example:

MyQueue queue = new MyQueue();queue.push(1);queue.push(2);  queue.peek();  // returns 1queue.pop();   // returns 1queue.empty(); // returns false

Notes:

You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is emptyoperations are valid.您必须仅使用一个堆栈的标准操作 - 这意味着只能从顶部切换到顶部，查看/弹出，大小，并且空操作是有效的。

Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.根据您的语言，本机可能不支持堆栈。您可以使用列表或双端队列（双端队列）来模拟堆栈，只要您只使用堆栈的标准操作即可。

You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).您可以假设所有操作都是有效的（例如，不会在空队列上调用pop或Peek操作）。

解答：

1 class MyQueue { 2      3     private Stack
     
       input; 4     private Stack
      
        output; 5      6     /** Initialize your data structure here. */ 7     public MyQueue() { 8         input = new Stack<>(); 9         output = new Stack<>();10     }11     12     /** Push element x to the back of queue. */13     public void push(int x) {14         input.push(x);15     }16     17     /** Removes the element from in front of queue and returns that element. */18     public int pop() {19         move();20         return output.pop();21     }22     23     /** Get the front element. */24     public int peek() {25         move();26         return output.peek();27     }28     29     /** Returns whether the queue is empty. */30     public boolean empty() {31         return input.isEmpty()&&output.isEmpty();32     }33     34     private void move() {35         if (output.isEmpty())36             while (!input.isEmpty()) 37                 output.push(input.pop());38     }39 }40 41 /**42  * Your MyQueue object will be instantiated and called as such:43  * MyQueue obj = new MyQueue();44  * obj.push(x);45  * int param_2 = obj.pop();46  * int param_3 = obj.peek();47  * boolean param_4 = obj.empty();48  */