input: def get_max_min(lst):
      """ Returns a tuple containing the maximum and minimum values in a given list.
      >>> get_max_min([1, 2, 3, 4, 5])
      (1, 5)
      >>> get_max_min([5, -3, 12, 8, 0])
      (-3, 12)
      """
output:     if not lst:
          return None
      max_val = min_val = lst[0]
      for num in lst:
          if num > max_val:
              max_val = num
          elif num < min_val:
              min_val = num
      return (min_val, max_val)