input: def get_max_min(lst):
      """Return the maximum and minimum values in a given list.
      >>> get_max_min([3, 5, 1, 2, 4])
      (1, 5)
      >>> get_max_min([100, -4, 999, 0])
      (-4, 999)
      """
output:     if not lst:
          return None
      max_val = max(lst)
      min_val = min(lst)
      return (min_val, max_val)