#class Node:
#  def __init__(self, value, next=None):
#    self.val = value
#    self.next = next

class Solution:
  def reverse(self, head, k):
    if k == 1:
      return head

    def sub_reverse(s, e):
      prev = None
      curr = s

      while curr != e:
        next_ = curr.next
        curr.next = prev
        prev = curr
        curr = next_

      return prev, start

    current = dummy = Node(-1, head)
    start = end = head

    while end:
      count = 0
      
      while end and count < k:
        end = end.next
        count += 1

      new_start, new_end = sub_reverse(start, end if count == k else None)

      current.next = new_start
      current = new_end
      start = end

    return dummy.next