Natural CountPath(City city)
{
if (city.HasValue())
{
return(city.PathCount);
}
if (visited[city])
{
// a cycle
foreach (City c in visitedCities)
{
// if any of the city in the cycle can reach warfare capital, then there's infinite path, otherwise 0
bool reachable = IsReachable(c, lastCity);
if (reachable)
{
return(Natural.Infinity);
}
if (c == city)
{
break;
}
}
return(0);
}
visited[city] = true;
visitedCities.Push(city);
ICollection <City> neighbours = city.Neighbours;
Natural totalCount = city == lastCity ? 1 : 0;
foreach (City n in neighbours)
{
totalCount = (totalCount + CountPath(n)) % MOD;
if (totalCount.IsInfinity())
{
break;
}
}
visitedCities.Pop();
visited[city] = false;
city.PathCount = totalCount;
return(totalCount);
}
void Run(TextReader rd)
{
string[] ln = rd.ReadLine().Split(' ');
N = int.Parse(ln[0]);
M = int.Parse(ln[1]);
visited = new bool[N + 1];
visited2 = new bool[N + 1];
visitedCities = new Stack <City>(N);
cities = new City[N + 1];
reachabletb = new int[N + 1];
for (int i = N; i > 0; i--)
{
City c = new City(i);
cities[i] = c;
}
firstCity = cities[1];
lastCity = cities[N];
for (int i = 0; i < M; i++)
{
ln = rd.ReadLine().Split(' ');
int from = int.Parse(ln[0]);
int to = int.Parse(ln[1]);
cities[from].Add(cities[to]);
}
rd.Dispose();
Natural count = CountPath(firstCity);
if (count.IsInfinity())
{
Console.WriteLine("INFINITE PATHS");
}
else
{
Console.WriteLine(count);
}
}
