Find the Greatest Common Divisor

Given two non-negative integers a and b, find and return their greatest common divisor (GCD).

The greatest common divisor is the largest positive integer that divides both numbers without leaving a remainder.

For example, the GCD of 24 and 36 is 12 because 12 is the largest integer that divides both numbers exactly.

You may use the Euclidean algorithm or another correct approach.

If one number is zero, the GCD is the other number. The input guarantees that both numbers will not be zero simultaneously.

Implement the function gcd(a, b) that returns the greatest common divisor.

0 <= a <= 1000000000 0 <= b <= 1000000000 At least one of a and b is greater than zero.