Factorial Challenge

In this exercise you are going to create a function that takes a number parameter and returns the factorial of it.

Factorial has a very specific definition. Learn more here. As an overview though, a factorial is the result of multiplying a sequence of descending numbers down to 0. Factorials are only defined for non-negative integer numbers. This definition includes zero. Though 0! is equal to 1, so treat it as an edge case.

Example

factorial(4) # => 24 (4 * 3 * 2 * 1)