odd_even_linkedlist.cpp

 ListNode* oddEvenList(ListNode* head) {
  if (!head) return nullptr;
  ListNode *odd = head, *even = head->next, *evenHead = even;
  while (odd->next && even->next) {
      odd->next = even->next;
      odd = odd->next;
      even->next = odd->next;
      even = even->next;
  }
  odd->next = evenHead;
  return head;
}