odd_even_linked_list.py

class Solution(object):
    def oddEvenList(self, head):
        if not head or not head.next:
            return head
        
        odd = head
        even = head.next
        even_head = even

        while even and even.next:
            odd.next = even.next
            odd = odd.next
            even.next = odd.next
            even = even.next
        
        odd.next = even_head

        return head