请用python写一个简短的递归代码，能够按升序对链表的元素进行排序

下面是一个使用归并排序的简单递归代码：

# 定义链表的节点
class ListNode:
  def __init__(self, val):
    self.val = val
    self.next = None

# 归并排序函数
def merge_sort(head):
  # 如果链表为空或只有一个元素，则直接返回
  if not head or not head.next:
    return head
  
  # 将链表分成两半
  slow, fast = head, head.next
  while fast and fast.next:
    slow = slow.next
    fast = fast.next.next
  mid = slow.next
  slow.next = None
  
  # 对两半分别进行排序
  left = merge_sort(head)
  right = merge_sort(mid)
  
  # 将两个有序链表合并
  return merge(left, right)

# 合并两个有序链表的函数
def merge(left, right):
  dummy = ListNode(0)
  cur = dummy
  while left and right:
    if left.val < right.val:
      cur.next = left
      left = left.next
    else:
      cur.next = right
      right = right.next
    cur = cur.next
  if left:
    cur.next = left
  if right:
    cur.next = right
  return dummy.next

希望这能帮到你！