input: Write a function to find the greatest common divisor (GCD) of two integers using recursion.
  assert gcd(48, 18) == 6
  assert gcd(54, 24) == 6
  assert gcd(101, 10) == 1
output: def gcd(a, b):
      if b == 0:
          return a
      else:
          return gcd(b, a % b)