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 1
queue.pop();   // returns 1
queue.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<Integer> input;
 4     private Stack<Integer> 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  */

详解：

1.队列：先进先出。 栈：先进后出

2.对于四个方法，队列的push和empty方法与栈相同，只是栈的pop和peek是针对队尾元素，而队列的pop和peek是针对队首元素

原文地址：https://www.cnblogs.com/chanaichao/p/9343791.html