Web Navigation
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/
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
1 #include<cstdio> 2 #include<iostream> 3 #include<string> 4 #include<stack> 5 using namespace std; 6 7 int main() 8 { 9 stack<string> back,forward; 10 string now,URL; 11 back.push("http://www.acm.org/"); 12 while(cin>>now) 13 { 14 if(now[0]==‘Q‘) 15 break; 16 else if(now[0]==‘V‘) 17 { 18 cin>>URL; 19 back.push(URL); 20 cout<<URL<<endl; 21 while(!forward.empty()) 22 forward.pop(); 23 } 24 else if(now[0]==‘B‘) 25 { 26 if(back.size()>1) 27 { 28 forward.push(back.top()); 29 back.pop(); 30 cout<<back.top()<<endl; 31 } 32 else 33 cout<<"Ignored"<<endl; 34 } 35 else 36 { 37 if(!forward.empty()) 38 { 39 back.push(forward.top()); 40 cout<<forward.top()<<endl; 41 forward.pop(); 42 } 43 else 44 cout<<"Ignored"<<endl; 45 } 46 } 47 return 0; 48 }