剑指offer第二版-9.用两个栈实现队列

/**
 * Copyright(C) 2019 Hangzhou Differsoft Co., Ltd. All rights reserved.
 *
 */
package com.java.offer;

import java.util.Stack;

/**
 * 用两个栈实现队列
 * 
 * @since 2019年2月25日 下午5:21:48
 * @author xuchao
 *
 */
public class P9_QueueWithTwoStacks {

    public static void main(String[] args) {
        Queue<Integer> queue = new Queue<>();
        queue.insert(1);
        queue.insert(2);
        queue.insert(3);
        System.out.println(queue.delete());
        System.out.println(queue.delete());
        queue.insert(4);
        System.out.println(queue.delete());
    }
}

class Queue<T> {
    private Stack<T> stack1 = new Stack<>();
    private Stack<T> stack2 = new Stack<>();

    public void insert(T data) {
        stack1.push(data);
    }
    
    public T delete() {
        if (!stack2.isEmpty()) {
            return stack2.pop();
        }else if(!stack1.isEmpty()) {
            while(!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
            return stack2.pop();
        }
        return null;
    }
}