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[0]) {
            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 0;
}