利用栈Stack实现队列(Queue)

时间:2022-08-30 17:39:06

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());
    }
}