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.
- Could you solve the problem in
O(1)extra memory space?
- You may not alter the values in the list’s nodes, only nodes itself may be changed.
Input: head = [1,2,3,4,5], k = 2 Output: [2,1,4,3,5]
Input: head = [1,2,3,4,5], k = 3 Output: [3,2,1,4,5]
Input: head = [1,2,3,4,5], k = 1 Output: [1,2,3,4,5]
Input: head = , k = 1 Output: 
- The number of nodes in the list is in the range
1 <= sz <= 5000
0 <= Node.val <= 1000
1 <= k <= sz