input: def max_num(numbers: List[int]) -> int:
      """ Find the maximum number in a given list of integers.
      >>> max_num([9, 2, 5, 6, 4, 1])
      9
      >>> max_num([3, 8, 7, 1])
      8
      """
output:     if not numbers:
          return None
      max_value = numbers[0]
      for num in numbers:
          if num > max_value:
              max_value = num
      return max_value