Take-home LeetCode-style questions. Once you start, you have 45 minutes to solve two problems. One of the questions was medium difficulty, the other was a bit hard but not too extreme.

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 nodes is not a multiple of k, then the remaining 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.