模拟网页的浏览Stack(POJ1028)

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

using namespace std;

stack<string> bstack,fstack;
string current;

void visit()
{
    if(current!="")
        bstack.push(current);
    cin>>current;
    cout<<current<<endl;
    while(!fstack.empty())  ///把以前的清空
        fstack.pop();
    return;
}

void back()
{
    if(bstack.empty())
    {
        cout<<"Ignored\n";
        return;
    }
    fstack.push(current);   ///把当前的放到以前的栈中
    current=bstack.top();   ///获得新的当前网页
    bstack.pop();
    cout<<current<<endl;
}

void forward()
{
    if(fstack.empty())
    {
        cout<<"Ignored\n";
        return;
    }
    bstack.push(current);   ///当前的网页给现在的栈
    current=fstack.top();   ///当前的网页变成以前的网页
    fstack.pop();
    cout<<current<<endl;
}

int main()
{
    string cmd;
    current="http://www.acm.org/";
    while(cin>>cmd&&cmd!="QUIT")
    {
        if(cmd=="VISIT")
            visit();
        else if(cmd=="FORWARD")
            forward();
        else if(cmd=="BACK")
            back();
    }
    return 0;
}