'''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 'n!'.

You are guaranteed an integer argument. For any values outside the positive range, 
return null, nil or None. 
For positive numbers a full length number is expected for example, 
return 25! = '15511210043330985984000000' as a String!

Note: 0! is always equal to 1. Negative values should return null;

For more on Factorials : http://en.wikipedia.org/wiki/Factorial'''


