Recursive greatest common divisor P42523

Write a recursive function that computes the greatest common divisor of two natural numbers $a$ and $b$ using the fast version of the Euclidean algorithm.

Interface

int gcd(int a, int b);

int gcd(int a, int b);

public static int gcd(int a, int b);

gcd(a, b)  # returns int

gcd(a: int, b: int) -> int

Precondition

Neither $a$ nor $b$ are negative, and at least one of them is strictly greater than zero.

Observation

You only need to submit the required procedure; your main program will be ignored.