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