POJ 1028

Web Navigation

Time Limit: 1000MS		Memory Limit: 10000K
Total Submissions: 23963		Accepted: 10692

Description

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

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.

Output

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.

Sample Input

VISIT http://acm.ashland.edu/

VISIT http://acm.baylor.edu/acmicpc/

BACK

BACK

BACK

FORWARD

VISIT http://www.ibm.com/

BACK

BACK

FORWARD

FORWARD

FORWARD

QUIT

Sample Output

http://acm.ashland.edu/

http://acm.baylor.edu/acmicpc/

http://acm.ashland.edu/

http://www.acm.org/

Ignored

http://acm.ashland.edu/

http://www.ibm.com/

http://acm.ashland.edu/

http://www.acm.org/

http://acm.ashland.edu/

http://www.ibm.com/

Ignored

//虽然给出了步骤，仍费了好大劲，此题很经典 

#include <iostream>

#include <string>

#include <stack>

using namespace std;

int main()

{

    int i,j,k;

    stack <string > b;

    stack <string > f;

    string s;

    string cur = "http://www.acm.org/";

    while(cin>>s)//cin以空格结束 

    {

        if(s=="QUIT")

            break;

        if(s[0]=='V')

        {

            b.push(cur);

            cin>>cur;

            cout<<cur<<endl;

            while(!f.empty())

                f.pop();

        }

        else if(s[0]=='B')

        {

            if(!b.empty())

            {

                //在visit语句中最后一个还没入任何站 

                f.push(cur);

                //这句话和下面那句一定不能搞反顺序 

                //Pop the page from the top of the backward stack, 

                //making it the new current page

                cur = b.top();

                b.pop();

                cout<<cur<<endl;//始终输出当前页 

            } 

            else

                cout<<"Ignored"<<endl;

        }

        else

        {

            if(!f.empty())

            {            

                b.push(cur);

                cur = f.top();

                f.pop();

                cout<<cur<<endl;//始终输出当前页 

            }

            else

                cout<<"Ignored"<<endl;

        }

    }

    return 0;

}