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.
You may not alter the values in the list's nodes, only nodes themselves 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: 
1 <= sz <= 5000
0 <= Node.val <= 1000
1 <= k <= sz
Follow-up: Can you solve the problem in O(1) extra memory space?
This is a tricky question, it requires knowledge of pointers, how we can reverse the node and call a recursive method with a new head pointer for subproblems. A detailed explanation is provided in the Video.
Time Complexity: O(n)
Space Complexity: O(n)