LeetCode 225. Implement Stack using Queues（使用队列来实现栈）

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原题网址：https://leetcode.com/problems/implement-stack-using-queues/

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).

思路：压栈的时候，使用一个临时队列存储最后压栈的元素（以便最先出队），然后将之前队列的内容逐个添加到临时队列，最后将临时队列覆盖回存放栈数据的队列，实现顺序反转。

源代码：

class MyStack {    private Queue<Integer> queue = new LinkedList<Integer>();    private Queue<Integer> buf = new LinkedList<Integer>();    // Push element x onto stack.    public void push(int x) {        buf.add(x);        while (!queue.isEmpty()) buf.add(queue.remove());        Queue<Integer> temp = queue;        queue = buf;        buf = temp;    }    // Removes the element on top of the stack.    public void pop() {        queue.remove();    }    // Get the top element.    public int top() {        return queue.peek();    }    // Return whether the stack is empty.    public boolean empty() {        return queue.isEmpty();    }}

时间复杂度：压栈O(n)，出栈、查看栈顶为O(1)，空间复杂度为O(n)

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