25.py

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def reverseKGroup(self, head, k):
        dummy = last = ListNode(0)
        cur = head
        while cur:
            first, cnt = cur, 1
            while cnt < k and cur:
                cur, cnt = cur.next, cnt + 1
            if cnt == k and cur:
                cur, prev = first, None
                for _ in range(k):
                    prev, cur.next, cur = cur, prev, cur.next
                last.next, last = prev, first
            else:
                last.next = first
        return dummy.next