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

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)