Write a function that will take an integer, n, as a parameter and will return an integer based on one of three cases:
if the input is positive (or zero) and even: return n/2
if the input is positive odd: return 3n+1 if the input is negative return 0 This is known as the collatz iteration.
Use the header: int collatz(int n)
Example: collatz(5) would return 16, collatz(16) would return 8,
collatz(-199) would return 0
