leetCode-225:Implement Stack using Queues

问题描述

要求用队列实现栈的操作。题目链接：点我

样例输入输出

输入：

[“MyQueue”, “push”, “push”, “peek”, “pop”, “empty”]
[[], [1], [2], [], [], []]

输出：

[null, null, null, 1, 1, false]

解释：

MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

输入：

[“MyQueue”, “empty”]

[[],[]]

输出：

[null, true]

问题解法

此题比较简单，直接使用队列的特性进行实现即可。代码如下

class MyStack {
    
    private Queue<Integer> queue;
    
    private Queue<Integer> queueBak;
    
    private Integer topNum = null;

    public MyStack() {
        queue = new LinkedList<>();
        queueBak = new LinkedList<>();
    }

    public void push(int x) {
        if (queue.size() == 0) {
            queueBak.offer(x);
        } else {
            queue.offer(x);
        }
        
        topNum = x;
    }

    public int pop() {
        topNum = null;
        if (queue.size() == 0) {
            while (queueBak.size() > 1) {
                topNum = queueBak.poll();
                queue.offer(topNum);
            }
            
            return queueBak.poll();
        }
        
        while (queue.size() > 1) {
            topNum = queue.poll();
            queueBak.offer(topNum);
        }
        
        return queue.poll();
    }

    public int top() {
        return topNum;
    }

    public boolean empty() {
        return queue.size() == 0 && queueBak.size() == 0;
    }
}

/**
 * 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();
 */