24. Reverse Nodes in k-Group

Given the head of a linked list, reverse the nodes of the list `k` at a time and return the modified list. If the number of remaining nodes is less than `k`, leave them as-is. For example, given [1,2,3,4,5] and k = 2, reverse each pair: [2,1,4,3,5]. For k = 3, reverse the first group of 3 and leave the last 2: [3,2,1,4,5]. You may not alter the values in the nodes, only the nodes themselves may be changed.