prob_25.py

# 25. Reverse Nodes in k-Group
# https://leetcode.com/problems/reverse-nodes-in-k-group/
# 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
        """
        dummy = jump = ListNode(-1)
        dummy.next = l = r = head
        
        while True:
            counter = 0

            while counter<k and r is not None:
                r = r.next
                counter += 1
            if counter == k:
                pre, cur = r, l
                for _ in range(k):
                    temp = cur.next
                    cur.next = pre
                    pre = cur
                    cur = temp
                jump.next = pre
                jump = l
                l = r
                
            else:
                return dummy.next