# Definition for singly-linked list.

# class ListNode(object):

#     def __init__(self, x):

#         self.val = x

#         self.next = None

class Solution(object):

    def reverseKGroup(self, head, k):

        """

        :type head: ListNode

        :type k: int

        :rtype: ListNode

        """

        if head == None:

            return None

        dummy=ListNode(0)

        dummy.next=head

        start=dummy

        while start.next:

            end=start

            for i in range(k-1):

                end=end.next

                if end.next == None:

                    return dummy.next

            res=self.reverse(start.next,end.next)

            start.next=res[0]

            start=res[1]

        return dummy.next

    def reverse(self,start,end):

        newHead=ListNode(0)

        newHead.next=start

        while newHead.next != end:

            tmp=start.next

            start.next=tmp.next

            tmp.next=newHead.next

            newHead.next=tmp

        return [end,start]