# Recursion 
# Def: "...is a method where the solution to a problem depends on solutions to smaller instance of the same problem.." - wiki
# TL;DR: a function that calls itself inside its body. 

# Recursive programs - Pseduocode 
function factorial: 
input: integer n such that n >= 0
output: n * (n-1) * (n-2) * ... * 1 = n!

    1. if n is 0,   return 1 
    2. else,        return ( n * factorial(n-1) )