232. Implement Queue using Stacks

Posted 2020-10-11 andywu

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

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.
• 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.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

题目含义：使用stack来实现queue的功能

 1 class MyQueue {
 2     Stack<Integer> input = new Stack();
 3     Stack<Integer> output = new Stack();
 4     /** Initialize your data structure here. */
 5     public MyQueue() {
 6         
 7     }
 8     
 9     /** Push element x to the back of queue. */
10     public void push(int x) {
11         input.push(x);     
12     }
13     
14     /** Removes the element from in front of queue and returns that element. */
15     public int pop() {
16              peek();
17         return output.pop();   
18     }
19     
20     /** Get the front element. */
21     public int peek() {
22         if (output.empty())
23             while (!input.empty())
24                 output.push(input.pop());
25         return output.peek();
26         
27     }
28     
29     /** Returns whether the queue is empty. */
30     public boolean empty() {
31          return input.empty() && output.empty();
32     }
33 }