input: Write a python function to find the greatest common divisor (GCD) of two numbers using recursion.
  assert gcd(12, 24) == 12
  assert gcd(7, 13) == 1
  assert gcd(20, 30) == 10
output: def gcd(a, b):
      if b == 0:
          return a
      else:
          return gcd(b, a % b)