# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def oddEvenList(self, head: Optional[ListNode]) -> Optional[ListNode]:
if not head:
return None
oddhead = head
evenhead = head.next
odd, even = oddhead, evenhead
while even and even.next:
odd.next = even.next
odd = odd.next
even.next = odd.next
even = even.next
odd.next = evenhead
return oddhead