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class Solution:
def oddEvenList(self, head: Optional[ListNode]) -> Optional[ListNode]:
if not head:
return None
odd = head
even = head.next
even_head = head.next
while even and even.next:
odd.next = odd.next.next
even.next = even.next.next
odd = odd.next
even = even.next
odd.next = even_head
return head
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