class Solution:
  def reverse(self, head, k):
    node = head
    new_head = None
    prev_end = None

    # while we still have remaining parts.
    while node:
      end = node
      prev = None
      count = 0

      # reverse the part.
      while node and count < k:
        count += 1
        next_node = node.next
        node.next = prev 
        prev = node 
        node = next_node
      begin = prev 

      # link the previous part end to the begin of the new part.
      if prev_end:
        prev_end.next = begin
      prev_end = end

      # set the new head of Linkedlist. it points to the first part begin.
      if not new_head:
        new_head = begin

      count = 0
    
    # We have to do this to prevent the Linkedlist from having circular loop at the end. 
    end.next = None

    return new_head