input: Write a function to find the greatest common divisor (GCD) of two numbers using an iterative approach.
  assert gcd(48, 18) == 6
  assert gcd(100, 25) == 25
  assert gcd(20, 8) == 4
output: def gcd(a, b):
      while b != 0:
          a, b = b, a % b
      return a