POJ1028 Web Navigation

题目来源：http://poj.org/problem?id=1028

题目大意：

　　模拟实现一个浏览器的“前进”和“回退”功能。由一个forward stack和一个backward stack实现。

　　打开浏览器时的正位于http://www.acm.org/。然后浏览器会接受下面四种命令：

　　BACK：将当前页面压入forward stack，将backward stack顶部页面弹出，成为当前页面。若当前backward stack为空，忽略该命令；

　　FORWARD：将当前页面压入backward stack，将forward stack顶部的页面弹出，作为当前页面。若当前forward stack为空，忽略该命令；

　　VISIT：将当前页面压入backward stack，将指定的URL作为当前页面；

　　QUIT：退出浏览器。

输入：命令的序列，以QUIT作为结束。

输出：如果当前页面发生了改变，输出改变后的当前页面，如果命令被忽略，输出“Ignored.”

---

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

---

题干已经把如何实现讲解得很清楚，照着写就可以了，要是不给提示，这倒会是道很有意思的题。

 //////////////////////////////////////////////////////////////////////////
 //        POJ1028 Web Navigation
 //        Memory: 264K        Time: 47MS
 //        Language: C++        Result: Accepted
 //////////////////////////////////////////////////////////////////////////

 #include <iostream>
 #include <string>
 #include <stack>

 using namespace std;

 stack<string> forwardStack;
 stack<string> backwardStack; 

 void clearForwardStack() {
     while (!forwardStack.empty()) {
         forwardStack.pop();
     }
 }

 int main() {
     string operation;
     string currentURL = "http://www.acm.org/";
     string nextURL;
     while (cin >> operation && operation != "QUIT") {

         switch (operation[]) {
             case 'V':
                 cin >> nextURL;
                 backwardStack.push(currentURL);
                 currentURL = nextURL;
                 cout << currentURL << endl;
                 clearForwardStack();
                 break;
             case 'B':
                 if (backwardStack.empty()) {
                     cout << "Ignored" << endl;
                 } else {
                     forwardStack.push(currentURL);
                     currentURL = backwardStack.top();
                     cout << currentURL << endl;
                     backwardStack.pop();
                 }
                 break;
             case 'F':
                 if (forwardStack.empty()) {
                     cout << "Ignored" << endl;
                 } else {
                     backwardStack.push(currentURL);
                     currentURL = forwardStack.top();
                     cout << currentURL << endl;
                     forwardStack.pop();
                 }
                 break;
         }
     }
     system("pause");
     return ;
 }