input: Write a function to calculate the greatest common divisor (GCD) of two given numbers by implementing the Euclidean algorithm.
  def findGCD(a, b): 
     if (b == 0): 
         return a
     return findGCD(b, a % b)
  assert findGCD(1000, 500)==500
  assert findGCD(48, 18)==6
  assert findGCD(81, 153)==9
  assert findGCD(7, 5)==1
  assert findGCD(0, 0)==0
output: def findGCD(a, b): 
     if (b == 0): 
         return a
     return findGCD(b, a % b)