利用栈Stack实现队列(Queue)

实现说明：

入队时，将元素压入s1;

出队时，判断s2是否为空，如不为空，则直接弹出顶元素；如为空，则将s1的元素逐个“倒入”s2，把最后一个元素弹出并出队;

这个思路，避免了反复“倒”栈，仅在需要时才“倒”一次。

package com.knowledgeStudy.threadLocal;

import java.util.Stack;

public class MyQueue {

    Stack<Object> s1 = new Stack<Object>();//入栈

    Stack<Object> s2 = new Stack<Object>();//出栈

    // Push element x to the back of queue.
    public void push(Object x) {
        s1.push(x);
    }

    // Removes the element from in front of queue.  
    public void pop() {
        if (!s2.isEmpty()) {
            s2.pop();
        } else {
            while (!s1.isEmpty()) {
                s2.push(s1.pop());
            }
            s2.pop();
        }
    }

    // Get the front element.  
    public Object peek() {
        if (!s2.isEmpty()) {
            return s2.peek();
        } else {
            while (!s1.isEmpty()) {
                s2.push(s1.pop());
            }
            return s2.peek();
        }
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return s1.empty() && s2.empty();
    }
   //测试
    public static void main(String[] args) {
        MyQueue queue = new MyQueue();
        queue.push(1);
        queue.pop();
        System.out.println(queue.empty());
    }
}