Lightoj1113——Discover the Web（STL，栈的使用）

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. You are asked to
implement this. The commands are:

1.      BACK: If the backward stack is empty, the command is ignored. Otherwise, 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.

2.      FORWARD: If the forward stack is empty, the command is ignored. Otherwise, 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.

3.      VISIT <url>: 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.

4.      QUIT: Quit the browser.

The browser initially loads the web page at the URL 'http://www.lightoj.com/'

Input

Input starts with an integer T (≤ 100), denoting the number of test cases.

Each case contains some commands. The keywords BACK, FORWARD, VISIT, and QUIT are all in uppercase. URLs have no whitespace and have at most 50 characters. The end of case is indicated by the QUIT command and it shouldn't be processed. Each case contains
at most 100 lines.

Output

For each case, print the case number first. For each command, print the URL of the current page (in a line) after the command is executed if the command is not ignored. Otherwise, print 'Ignored'.

Sample Input	Output for Sample Input
1 VISIT http://uva.onlinejudge.org/ VISIT http://topcoder.com/ BACK BACK BACK FORWARD VISIT http://acm.sgu.ru/ BACK BACK FORWARD FORWARD FORWARD QUIT	Case 1: http://uva.onlinejudge.org/ http://topcoder.com/ http://uva.onlinejudge.org/ http://www.lightoj.com/ Ignored http://uva.onlinejudge.org/ http://acm.sgu.ru/ http://uva.onlinejudge.org/ http://www.lightoj.com/ http://uva.onlinejudge.org/ http://acm.sgu.ru/ Ignored

这简直是STL中stack的完美运用，以后都不用麻烦自己去定义一个栈了

#include<stdio.h>
#include<algorithm>
#include<string.h>
#include<iostream>
#include<cmath>
#include<stack>
#define MAXN 1000010
using namespace std;
stack<string> backward;
stack<string> Forward;
int main()
{
string op,url;
int t,cnt=1;
scanf("%d",&t);
while(t--)
{
backward.push("http://www.lightoj.com/");
printf("Case %d:\n",cnt++);
while(1)
{
cin>>op;
if(op[0]=='B')
{
if(!backward.empty())
{
Forward.push(backward.top());
backward.pop();
if(backward.empty())
{
backward.push(Forward.top());
Forward.pop();
cout<<"Ignored"<<endl;
}
else
cout<<backward.top()<<endl;
}
}
if(op[0]=='F')
{
if(!Forward.empty())
{
backward.push(Forward.top());
Forward.pop();
cout<<backward.top()<<endl;
}
else
cout<<"Ignored"<<endl;
}
if(op[0]=='V')
{
cin>>url;
backward.push(url);
while(!Forward.empty())
Forward.pop();
cout<<url<<endl;
}
if(op[0]=='Q')
break;
}
while(!Forward.empty())
Forward.pop();
while(!backward.empty())
backward.pop();
}
return 0;
}