toj 1196 栈的运用（模拟浏览器）

两个栈来模拟浏览器的操作。

#include <iostream>
#include <string>
#include <stack>
using namespace std;

stack<string> forward;
stack<string> back;

int main ()
{
    string cur("http://www.acm.org/"), tmp;
    while ( cin >> tmp )
    {
        if ( tmp == "QUIT" )
        {
            break;
        }
        else if ( tmp == "BACK" )
        {
            if ( back.empty() )
            {
                cout << "Ignored" << endl;
            }
            else
            {
                forward.push(cur);
                cur = back.top();
                back.pop();
                cout << cur << endl;
            }
        }
        else if ( tmp == "FORWARD" )
        {
            if ( forward.empty() )
            {
                cout << "Ignored" << endl;
            }
            else
            {
                back.push(cur);
                cur = forward.top();
                forward.pop();
                cout << cur << endl;
            }
        }
        else if ( tmp == "VISIT" )
        {
            back.push(cur);
            cin >> cur;
            while ( !forward.empty() )
            {
                forward.pop();
            }
            cout << cur << endl;
        }
    }
    return 0;
}