answer.py

#!/usr/bin/env python3

#-------------------------------------------------------------------------------
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
#-------------------------------------------------------------------------------
# Solution
#-------------------------------------------------------------------------------

class Solution:
    def reverseKGroup(self, head, k):
        """
        :type head: ListNode
        :type k: int
        :rtype: ListNode
        """
        dummy = jump = ListNode(0)
        dummy.next = l = r = head

        while True:
            count = 0
            while r and count < k:   # use r to locate the range
                r = r.next
                count += 1
            if count == k:  # if size k satisfied, reverse the inner linked list
                pre, cur = r, l
                for _ in range(k):
                    cur.next, cur, pre = pre, cur.next, cur  # standard reversing
                jump.next, jump, l = pre, l, r  # connect two k-groups
            else:
                return dummy.next
#-------------------------------------------------------------------------------