LeetCode 225题用队列实现栈（Implement St

链接

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

思路

首先演示push（）操作；
将元素依次进入队1，进入时用top元素保存当前进入的元素；
如下图：

push操作的演示

然后演示pop（）操作；
先将除队1中的最后一个元素出队并进入队2，入队2时用top存储入队元素；
再将队列1和队列2进行互换即可。
如下图：

pop操作的演示

代码

import java.util.LinkedList;
import java.util.Queue;

class MyStack {

    private Queue<Integer> queue_1 = new LinkedList<>();
    private Queue<Integer> queue_2 = new LinkedList<>();
    private int top;
    /** Initialize your data structure here. */
    public MyStack() {

    }

    /** Push element x onto stack. */
    public void push(int x) {
        queue_1.add(x);
        top = x;
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        if(empty()){
            throw new RuntimeException("MyStack空了!");
        }
        while(queue_1.size() > 1){
            top = queue_1.remove();
            queue_2.add(top);
        }
        int last_ele = queue_1.remove();
        Queue<Integer> queue_temp = queue_1;
        queue_1 = queue_2;
        queue_2 = queue_temp;
        return last_ele;
    }

    /** Get the top element. */
    public int top() {
        if(empty()){
            throw new RuntimeException("MyStack空了!");
        }
        return top;
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue_1.isEmpty() && queue_2.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();
 */

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