Mathematically, the formula for the factorial is as follows. If n is an integer greater than or equal to 1, then
`n ! = n ( n - 1)( n - 2)( n - 3) ... (3)(2)(1)`

If `p = 0`

, then `p ! = 1`

by convention.

def factorial(n): if n == 0: return 1 else: return n * factorial(n-1)

Hope this helps