Reverse Nodes in k-Group

Reverse Nodes in k-Group Given a linked list, reverse the nodes of the list k at a time and return its modified list. You may not alter the values in the nodes, only nodes themselves may be changed. Only constant memory is allowed. Example: Input: 1->2->3->4->5 with k = 2 Output: 2->1->4->3->5 Note: If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is. Complexity allowed: O(n) Constraints: The number of nodes in the list is in the range [0, 5000]. 1 <= k <= n Example Input 1->2->3->4->5; k = 2