Hi,

This is my code for merge sorting a linked list. My code works for the case of a linked list of two elements, but in the case of four elements, I got a never-ending loop. Could anyone point out what my problem is? This is from LxxxCode Problem 148.

```
# Definition for singly-linked list.
class ListNode:
def __init__(self, val=0, next=None):
self.val = val
self.next = next
class Solution:
#def sortList(self, head: Optional[ListNode]) -> Optional[ListNode]:
def sortList(self, head):
def midpoint(head): # Find the midpoint of a linked list given the head; the end of the first half
fast = slow = head
while(fast and fast.next):
slow = slow.next
fast = fast.next.next
return slow # Note that slow.next is the head of the second half
def twolist(head): # break the linked list into the first half and the second half; return heads of the first half and the second half
mid1 = midpoint(head)
head2 = mid1.next
mid1.next = None # separte the first half from the second half
return head, head2
def merge_sorted(head1, head2): # merge two sorted L.L.
pt = dummy = ListNode(-1) # pt: pointer
if head1.val < head2.val:
dummy.next = head1
else:
dummy.next = head2
pt = dummy.next
while head1 and head2:
if head1.val < head2.val:
pt.next = head1
head1 = head1.next
else:
pt.next = head2
head2 = head2.next
pt = pt.next
if head1:
pt.next = head1
else:
pt.next = head2
return dummy.next
# start the merge sort function
if not head: # 0 element
return head
if not head.next: # 1 element
return head
if not head.next.next: # 2 elements
if head.val < head.next.val: # right order
return head
else: # wrong order
dummy = ListNode(-1)
dummy.next = head.next
head.next.next = head
head.next = None
return dummy.next
# 3 elements or more
first_half_head, second_half_head = twolist(head)
first_half_sorted_head = self.sortList(first_half_head)
second_half_sorted_head = self.sortList(second_half_head)
return merge_sorted(first_half_sorted_head, second_half_sorted_head)
# Try this
sol1 = Solution()
e1 = ListNode(4)
e2 = ListNode(2)
e3 = ListNode(1)
e4 = ListNode(3)
e1.next = e2
e2.next = e3
e3.next = e4
es = sol1.sortList(e1)
print(es.val)
print(es.next.val)
print(es.next.next.val)
print(es.next.next.next.val)
```