input: Write a function to find the highest common factor (HCF) of two numbers.
  assert highestCommonFactor(54, 24) == 6
  assert highestCommonFactor(48, 180) == 12
  assert highestCommonFactor(101, 103) == 1
output: def highestCommonFactor(a, b):
      while b:
          a, b = b, a % b
      return a