input: def max_num(lst: 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, 3, 3, 3])
      3
      """
output:     if not lst:
          return None
      max_value = lst[0]
      for num in lst:
          if num > max_value:
              max_value = num
      return max_value