002-由两个栈组成的队列

package com.my.suanfa;

import java.util.Stack;

/*
 * 由两个栈组成的队列
 * 向stackPop中压入数据的时机有很多。在这里选择出队和查看队头元素时压入
 * 但是压入数据要满足两条：
 * 1.只能在stackPop为空时才能压入数据
 * 2.如果需要压入数据，则必须将stackPush中的数据全部压入
 * */
public class TwoStacksQueue {
	//声明变量
	private Stack<Integer> stackPush;
	private Stack<Integer> stackPop;
	//初始化变量
	public TwoStacksQueue() {
		this.stackPush = new Stack<Integer>();
		this.stackPop = new Stack<Integer>();
	}
	//入队操作
	public void add(int pushInt) {
		this.stackPush.push(pushInt);
	}
	//出队操作
	public int poll() {
		//判断队列是否为空
		if(this.stackPush.isEmpty() && this.stackPop.isEmpty()) {
			throw new RuntimeException("your queue is empty!");
		} else if(this.stackPop.isEmpty()) {
			while(!this.stackPush.isEmpty()) {
				this.stackPop.push(this.stackPush.pop());
			}
		}
		return this.stackPop.pop();
	}
	//查看队列头的元素
	public int peek() {
		if(this.stackPop.isEmpty() && this.stackPush.isEmpty()) {
			throw new RuntimeException("your queue is empty!");
		} else if(this.stackPop.isEmpty()) {
			while(!this.stackPush.isEmpty()) {
				this.stackPop.push(this.stackPush.pop());
			}
		}
		return this.stackPop.peek();
	}
}