Factorial
Factorial is the product of all positive integers less than or equal to a given positive integer and denoted by that integer and an exclamation point.
n! = n * (n-1) * (n-2) * (n-3) ... 3 * 2 * 1
For example
5! = 5 * 4 * 3 * 2 * 1 = 120
Just like the Fibonacci sequence, this is one of the basic forms of exercise used in programming and is commonly used to introduce recursion.
