A fast factorial implementation using the divide and conquer method.

FastFactorial.cs

public static class FastFactorial
{
	public static BigInteger Factorial(BigInteger n)
	{
		return MultiplyRange(2, n);
	}

	private static BigInteger MultiplyRange(BigInteger from, BigInteger to)
	{
		var diff = to - from;
		if (diff == 1) { return from * to; }
		if (diff == 0) { return from; }

		BigInteger half = (from + to) / 2;
		return BigInteger.Multiply(MultiplyRange(from, half), MultiplyRange(half + 1, to));
	}
}