Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.
k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.
Example:
Given this linked list: 1->2->3->4->5
For k = 2, you should return: 2->1->4->3->5
For k = 3, you should return: 3->2->1->4->5
Note:
       Only constant extra memory is allowed.
       You may not alter the values in the list's nodes, only nodes itself may be changed.
# Definition for singly-linked list.
 # class ListNode:
 #     def __init__(self, x):
 #         self.val = x
 #         self.next = None
 
 class Solution:
     def reverseKGroup(self, head, k):
         """
         :type head: ListNode
         :type k: int
         :rtype: ListNode
         """
         if head == None:
             return None
         if k == 1:
             return head
         res = []
         while(head!=None):
             res.append(head.val)
             head = head.next
         lens = len(res)
         res1 = []
         start = 0
         while(start+k <= lens):
             b = res[start:start+k]
             c = list(reversed(b))
             res1 += c
             start += k
         b = res[start:lens]
         res1 += b
         return res1
         
         
# Definition for singly-linked list.
 # class ListNode:
 #     def __init__(self, x):
 #         self.val = x
 #         self.next = None
 
 class Solution:
     def reverseKGroup(self, head, k):
         """
         :type head: ListNode
         :type k: int
         :rtype: ListNode
         """
         if head == None:
             return None
         if k == 1:
             return head
         res = []
         while(head!=None):
             res.append(head.val)
             head = head.next
         lens = len(res)
         res1 = []
         start = 0
         while(start+k <= lens):
             b = res[start:start+k]
             c = list(reversed(b))
             res1 += c
             start += k
         b = res[start:lens]
         res1 += b
         return res1
         
         