POJ 1028-Web Navigation Web Navigation Time Limit: 1000MS Memory Limit: 10000K Total Submissions: 29660 Accepted: 13249 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 Source East Central North America 2001
水题一个。使用STL自带stack快速解决。有一个小细节需要注意：
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.如果栈为空，则这个命令被忽略。被忽略的意思是就当没出现这个命令，自然也就不需要执行前半部分的操作了。
BACK: Push the current page on the top of the forward stack. FORWARD: Push the current page on the top of the backward stack.完整代码如下： [cpp] #include<iostream> #include<stack> #include<string> using namespace std; int main() { //freopen("input.txt","r",stdin); string current_url="http://www.acm.org/"; stack<string> forward_url,backward_url; string command; while(cin>>command&&command!="QUIT") { if(command=="VISIT") { backward_url.push(current_url); cin>>current_url; cout<<current_url<<endl; while(!forward_url.empty()) { forward_url.pop(); } } else if(command=="BACK") { //forward_url.push(current_url);//If the backward stack is empty, the command is ignored忽略的话，这一句也不能执行 if(!backward_url.empty()) { forward_url.push(current_url); current_url=backward_url.top(); backward_url.pop(); cout<<current_url<<endl; } else { cout<<"Ignored"<<endl; } } else if(command=="FORWARD") { //backward_url.push(current_url); if(!forward_url.empty()) { backward_url.push(current_url); current_url=forward_url.top(); forward_url.pop(); cout<<current_url<<endl; } else { cout<<"Ignored"<<endl; } } } return 0; } [/cpp] 代码提交AC，用时0MS，内存200K。]]>