input: Write a python function to compute the greatest common divisor (GCD) of two numbers.
  assert computeGCD(48, 18) == 6
  assert computeGCD(101, 103) == 1
  assert computeGCD(56, 98) == 14
output: def computeGCD(a, b):
      while b:
          a, b = b, a % b
      return a