Reverse Nodes in K-Group

The problem Given the head of a linked list, reverse the nodes of the list k at a time, and return the 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 the left-out nodes at the end should remain as they are. You may not alter the values in the list’s nodes, only nodes themselves may be changed. ...