LeetCode - 225. Implement Stack Using Queues

Description

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.
Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);
stack.top(); // returns 2
stack.pop(); // returns 2
stack.empty(); // returns false
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).

Code

借助队列来实现栈，实现出栈、入栈、返回栈顶元素和判断栈是否为空几个方法
我们首先看下栈的性质：先进后出
而队列的性质：先进先出
如果只借助一个队列，我们在加入元素时，要做的就是把队列中的元素反转，这样在pop元素时，能够保证先进入的元素后出，满足栈的性质

package com.jie.leetcode;

import java.util.LinkedList;
import java.util.Queue;

/**
 * Created by lujie on 2018/10/18.
 */
public class MyStack {

    private Queue<Integer> queue = new LinkedList<>();
    /** Initialize your data structure here. */
    public MyStack() {

    }

    /** Push element x onto stack. */
    public void push(int x) {
        queue.add(x);
        for(int i = 1;i < queue.size(); i++){
            queue.add(queue.poll());
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return queue.poll();
    }

    /** Get the top element. */
    public int top() {
        return queue.peek();
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue.isEmpty();
    }
}