implement-stack-using-queues（easy，但也有思考价值）

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<Integer>();
        queues[1] = new LinkedList<Integer>();
        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
", ret);
        solution.pop();
        ret = solution.top();
        System.out.printf("ret:%d
", ret);
        solution.pop();
        ret = solution.top();
        System.out.printf("ret:%d
", ret);

        System.out.println();

    }

}