Kata

In mathematics, the factorial of integer n is written as n!. It is equal to the product of n and every integer preceding it. For example: 5! = 1 x 2 x 3 x 4 x 5 = 120

Your mission is simple: write a function that takes an integer n and returns the value of n!.

You are guaranteed an integer argument. For any values outside the non-negative range, return null, nil or None (return an empty string "" in C and C++). For non-negative numbers a full length number is expected for example, return 25! = "15511210043330985984000000" as a string.

For more on factorials, see http://en.wikipedia.org/wiki/Factorial

NOTES:

• The use of BigInteger or BigNumber functions has been disabled, this requires a complex solution

• I have removed the use of require in the javascript language.