【剑指offer】 面试题7： 用两个栈实现队列

题目描述：

用两个栈来实现一个队列，完成队列的Push和Pop操作。
队列中的元素为int类型

package com.offer.chapter_2;

import java.util.Scanner;

class Stack<E> {
	private Object[] elem = new Object[100001];
	private int top = 0;
	
	public boolean isEmpty() {
		return top == 0;
	}
	
	public int size() {
		return top;
	}
	
	public E pop() {
		return (E)elem[--top];
	}
	
	public void push(E val) {
		elem[top++] = val;
	}
}

class Queue<E> {
	private Stack<E> stackPush = new Stack<E>();
	private Stack<E> stackPop = new Stack<E>();
	
	private int size() {
		return stackPush.size() + stackPop.size();
	}
	
	public void push(E val) {
		stackPush.push(val);
	}
	
	public boolean isEmpty() {
		return size() == 0;
	}
	
	public E pop() {
		if(!stackPop.isEmpty()) {
			return stackPop.pop();
		} 
		
		while(!stackPush.isEmpty()) {
			stackPop.push(stackPush.pop());
		}
		
		return stackPop.pop();
	}
}

/**
 * @author hadoop
 *
 *	用两个栈来实现一个队列，完成队列的Push和Pop操作。
 *
 */
public class Interviews_7 {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		
		int num = scanner.nextInt();
		scanner.nextLine();
		Queue<Integer> queue = new Queue<Integer>();
		
		while(num -- > 0) {
			String line = scanner.nextLine();
			String strs[] = line.split(" ");
			if(strs[0].equals("PUSH")) {
				queue.push(Integer.parseInt(strs[1]));
			} else if(strs[0].equals("POP")) {
				if(queue.isEmpty()) {
					System.out.println("-1");
				} else {
					System.out.println(queue.pop());
				}
			}
		}
	}
}