input: def max_num(list: List[int]) -> int:
      """ Return the maximum number in the given list.
      >>> max_num([9, 2, 5, 6, 4, 1])
      9
      >>> max_num([10, 22, 3, 40, 5])
      40
      """
output:     if not list:
          return None
      max_value = list[0]
      for num in list:
          if num > max_value:
              max_value = num
      return max_value