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

还有种方法，就是利用同一个队列，知道队列长度前提下，把内容从头到尾，再向尾部依次重推一下。

package com.company;import java.util.Deque;import java.util.LinkedList;import java.util.Queue;class Solution {    Queue[] queues;    int cur;    Solution() {        queues = new LinkedList[2];        queues[0] = new LinkedList
     
      ();        queues[1] = new LinkedList
      
       ();        cur = 0;    }    // Push element x onto stack.    public void push(int x) {        queues[cur].offer(x);    }    // Removes the element on top of the stack.    public void pop() {        change(true);    }    // Get the top element.    public int top() {        return change(false);    }    private int change(boolean pop) {        int next = (cur + 1) % 2;        int num = (int)queues[cur].poll();        while (!queues[cur].isEmpty()) {            queues[next].offer(num);            num = (int)queues[cur].poll();        }        if (!pop) {            queues[next].offer(num);        }        cur = next;        return num;    }    // Return whether the stack is empty.    public boolean empty() {        return queues[cur].isEmpty();    }}public class Main {    public static void main(String[] args) throws InterruptedException {        System.out.println("Hello!");        Solution solution = new Solution();        // Your Codec object will be instantiated and called as such:        solution.push(1);        solution.push(2);        solution.push(3);        int ret = solution.top();        System.out.printf("ret:%d\n", ret);        solution.pop();        ret = solution.top();        System.out.printf("ret:%d\n", ret);        solution.pop();        ret = solution.top();        System.out.printf("ret:%d\n", ret);        System.out.println();    }}