Algorithms

// Calculate the least significant (non-zero) digits of large factorials.
// By Robert C. Cartaino
// Posted via http://www.codeproject.com
// 02-July-2008

using System;

class Program
{
    const ulong Target = 1000000;       // This is the number you are trying to find the factorial of.
    const ulong RoundOff = 10000000000; // This rounds off the answer so the intermediate results don't overflow.

    static void Main(string[] args)
    {
        ulong factorial = 1;

        // Iterate through all numbers up to 'Target', multiply by each to find the factorial of 'Target'.
        for (ulong n = 1; n <= Target; n++)
        {
            factorial *= n;

            // Remove the trailing zeros.
            while (factorial % 10 == 0)
            {
                factorial /= 10;
            }

            // We only need the right-most digits.
            factorial %= RoundOff;

            //Console.WriteLine("{0}! = {1}", n, factorial);
        }

        Console.WriteLine("{0}! (truncated) = {1}", Target, factorial);
    }
}