input: Write a python function to find the greatest common divisor (GCD) of two numbers using recursion.
  assert gcd(8, 12) == 4
  assert gcd(48, 18) == 6
  assert gcd(101, 103) == 1
output: def gcd(a, b):
      if b == 0:
          return a
      else:
          return gcd(b, a % b)