class Node:
   def __init__(self, data):
      self.data = data
      self.next = None

class LinkedList_structure:
   def __init__(self):
      self.head = None
      self.last_node = None

   def add_vals(self, data):
      if self.last_node is None:
         self.head = Node(data)
         self.last_node = self.head
      else:
         self.last_node.next = Node(data)
         self.last_node = self.last_node.next

def print_middle_val(my_list):
   curr = my_list.head
   my_len = 0
   while curr:
      curr = curr.next
      my_len = my_len + 1

   curr = my_list.head
   for i in range((my_len - 1)//2):
      curr = curr.next

   if curr:
      if my_len % 2 == 0:
         print('两个中间元素是{}和{}'.format(curr.data, curr.next.data))
      else:
         print('中间元素是{}。'.format(curr.data))
   else:
      print('列表为空')

my_instance = LinkedList_structure()

my_list = input('输入链表元素... ').split()
for elem in my_list:
   my_instance.add_vals(int(elem))

print_middle_val(my_instance)

输入链表元素... 56 23 78 99 34 11
两个中间的元素是78和99。