225. Implement Stack using Queues

Posted 2020-10-11 andywu

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

题目含义：使用queue来实现stack的功能

 

 1 class MyStack {
 2 
 3     private Queue<Integer> p = new LinkedList<>();
 4 
 5     /** Initialize your data structure here. */
 6     public MyStack() {
 7 
 8     }
 9 
10     /** Push element x onto stack. */
11     public void push(int x) {
12         p.add(x);
13         for (int i=1;i<p.size();i++)
14         {
15             p.add(p.poll());
16         }
17     }
18 
19     /** Removes the element on top of the stack and returns that element. */
20     public int pop() {
21         return p.poll();
22     }
23 
24     /** Get the top element. */
25     public int top() {
26         return p.peek();
27     }
28 
29     /** Returns whether the stack is empty. */
30     public boolean empty() {
31         return p.isEmpty();
32     }
33 }