这样……
#include < iostream >
#include < string >
using namespace std ;

int main ( )
{
	string order , url[100] ;
	url[0] = "http://www.acm.org/" ;
	cin >> order ;
	for ( int i = 0 , j = 0 ; order != "QUIT" ; cin >> order )
	{
		if ( order == "VISIT" )
		{
			i++ ;
			j = i ;
			cin >> url[i] ;
			cout << url[i] << endl ;
		}
		if ( order == "BACK" && i - 1 >= 0 )
		{
			i-- ;
			cout << url[i] << endl ;
			continue ;
		}
		if ( order == "BACK" && i <= 0 )
		{
			cout << "Ignored" << endl ;
		}
		if ( order == "FORWARD" && i + 1 <= j )
		{
			i++ ;
			cout << url[i] << endl ;
			continue ;
		}
		if ( order == "FORWARD" && i >= j )
		{
			cout << "Ignored" << endl ;
		}
	}
	return 0 ;
}