# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def deleteMiddle(self, head: Optional[ListNode]) -> Optional[ListNode]:
slow = fast = head
prev = ListNode(0)
prev.next = slow
while fast.next and fast.next.next:
prev = slow
slow = slow.next
fast = fast.next.next
if fast.next and not fast.next.next:
# even - delete slow
if slow and slow.next and slow.next.next: # check head
slow.next = slow.next.next
else:
# odd - delete prev
if prev and prev.next and prev.next.next: # check head
prev.next = prev.next.next
if prev.next == head:
if head.next is None:
return None
head.next = None
return head