def reverse_k(head, k):
  pointers = []
  it = head
  for _ in range(k):
    if it is None: return head
    pointers.append(it)
    it = it.next
  
  last = reverse_k(pointers[-1].next, k)
  for p1, p2 in zip(pointers, [last] + pointers):
    p1.next = p2

  return pointers[-1]