PROJECT EULER #15

Link to page

Starting in the top left corner of a 22 grid, there are 6 routes (without backtracking) to the bottom right corner.

How many routes are there through a 2020 grid?

I used this file to generate BigInt

using System;

namespace ProjectEuler
{
   class Program
   {
       static void Main()
       {
           //Problem 15
           DateTime start = DateTime.Now;
           //This is a staircase walk problem
           //For a (m x n) grid the answer is
           //(m + n)! / m!n! = 40!/(20!*20!)
           BigInt answer = Factorial(40)/(Factorial(20)*Factorial(20));
           Console.WriteLine(answer);
           TimeSpan time = DateTime.Now-start;
           Console.WriteLine("This took {0}", time);
           Console.ReadKey();
       }
       public static BigInt Factorial(BigInt n)
       {
           return ((n == 0) ? 1 : n*Factorial(n - 1));
       }
   }
}