leetcode Reverse Nodes in k-Group python

# 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]

@link http://www.cnblogs.com/zuoyuan/p/3785555.html