LeetCode 225 Implement Stack using Queues

题目：

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).

题目链接

题意：

要求实现一个基本栈的操作：push(int)，pop()，top()，empty()，只能使用标准Queue中的操作。

在java中，Queue是一个接口，LinkedList类实现了这个接口，我们可以使用两个LinkedList来模拟Stack的操作，因为queue是先进先出，所以每次pop和top查询时利用两个queue，一个queue；不断pop，只到最后一项，然后返回最后一项，并将其他的项存入另一个queue中，来模拟stack的后进先出。

代码如下：

class MyStack {    private Queue<Integer> queue, temp;    public MyStack() {        queue = new LinkedList<>();        temp = new LinkedList<>();    }    public void push(int x) {        queue.offer(x);    }    public int pop() {        temp.clear();        while (queue.size() > 1) {            temp.offer(queue.poll());        }        int x = queue.poll();        queue = new LinkedList<>(temp);        return x;    }    public int top() {        temp = new LinkedList<>(queue);        while (temp.size() > 1) {            temp.poll();        }        return temp.poll();    }    public boolean empty() {        return queue.isEmpty();    }}/** * Your MyStack object will be instantiated and called as such: * MyStack obj = new MyStack(); * obj.push(x); * int param_2 = obj.pop(); * int param_3 = obj.top(); * boolean param_4 = obj.empty(); */