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.
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).

使用queue结构进行实现stack结构，需要处理最后面的一个元素。使用该元素时，要注意两个queue的相互交换等一些问题的处理技巧。

public class MyStack {    Queue<Integer> queue1;    Queue<Integer> queue2;    /** Initialize your data structure here. */    public MyStack() {        queue1 = new LinkedList<Integer>();        queue2 = new LinkedList<Integer>();    }    private void p2p() {        int len = queue1.size() - 1;        for (int i = 0; i < len; i++) {            queue2.offer(queue1.poll());        }    }    private void swapQueue() {        Queue temp = queue1;        queue1 = queue2;        queue2 = temp;    }    /** Push element x onto stack. */    public void push(int x) {        queue1.offer(x);    }    /** Removes the element on top of the stack and returns that element. */    public int pop() {        p2p();        int temp = queue1.poll();        swapQueue();        return temp;    }    /** Get the top element. */    public int top() {        p2p();        int temp = queue1.poll();        swapQueue();        queue1.offer(temp);        return temp;    }    /** Returns whether the stack is empty. */    public boolean empty() {        return queue1.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(); */