input: Write a function to find the greatest common divisor (GCD) of two numbers. If either of the numbers is zero, return the other number.
  def findGCD(a, b): 
     if (b == 0): 
         return a
     return findGCD(b, a % b)
  assert findGCD(1000, 500) == 500
  assert findGCD(0, 500) == 500
  assert findGCD(1000, 0) == 1000
  assert findGCD(14, 21) == 7
  assert findGCD(0, 0) == 0
output: def findGCD(a, b): 
     if (b == 0): 
         return a
     return findGCD(b, a % b)