Standard web browsers contain features to move backward and forward among the pages recently visited. One way to implement these features is to use two stacks to keep track of the pages that can be reached by moving backward and
forward. In this problem, you are asked to implement this.

The following commands need to be supported:

BACK: Push the current page on the top of the forward stack. Pop the page from the top of the backward stack, making it the new current page. If the backward stack is empty, the command is ignored.


FORWARD: Push the current page on the top of the backward stack. Pop the page from the top of the forward stack, making it the new current page. If the forward stack is empty, the command is ignored.


VISIT : Push the current page on the top of the backward stack, and make the URL specified the new current page. The forward stack is emptied.


QUIT: Quit the browser.

Assume that the browser initially loads the web page at the URL http://www.acm.org/

Input is a sequence of commands. The command keywords BACK, FORWARD, VISIT, and QUIT are all in uppercase. URLs have no whitespace and have at most 70 characters. You may assume that no problem instance requires more than 100 elements
in each stack at any time. The end of input is indicated by the QUIT command.

For each command other than QUIT, print the URL of the current page after the command is executed if the command is not ignored. Otherwise, print "Ignored". The output for each command should be printed on its own line. No output
is produced for the QUIT command.

二、题解

        这是挺水的一道题，首先题目就挺简单的（英语除外）还告诉我们怎么一步步做。要是看懂了英文题目完全可以照着做就行了。但是我这道题RE4次，最后发现是有几句if判断省了括号。以后不能偷懒了，改写的一定要写上。

三、题解

import java.util.Scanner;

import java.util.Stack;

public class Main {

	public static void main(String[] args) {

		Scanner sc=new Scanner(System.in);

		Stack<String> forward=new Stack<String>();

		Stack<String> backward=new Stack<String>();

		String current="http://www.acm.org/";

		String order="";

		String display="";

		while(!(order=sc.next()).equals("QUIT")){

			if(order.equals("VISIT")){

				backward.push(current);

				current=sc.next();

				forward.clear();

				display=current;

			}else if(order.equals("BACK")){

				if(backward.empty()){

					display="Ignored";

				}

				else{

					forward.push(current);

					current=backward.pop();

					display=current;

				}

			}else if(order.equals("FORWARD")){

				if(forward.empty()){

					display="Ignored";

				}

				else{

					backward.push(current);

					current=forward.pop();

					display=current;

				}

			}

			System.out.println(display);

		}

	}

}

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