Outline a method to concatenate two singly linked lists without allocating new nodes.

• A. Create a new node for each element from the second list and append.
• B. Insert the head of the second list into the middle of the first list.
• C. Link the tail of the first list to the head of the second; update tail if you track it; ensure no cycles.
• D. Replace the head of the first list with the head of the second.

Answer: (C)

To concatenate two singly linked lists without allocating new nodes, you connect the end of the first list to the beginning of the second. This means link the tail node of the first list to the head node of the second list. If you track the tail, this is an O(1) operation; after linking, the new tail becomes the tail of the second list. Handle edge cases: if the first list is empty, the result is the second list; if the second list is empty, the result is the first list. Ensure you don’t create cycles by making sure the last node of the first list doesn’t point somewhere unexpected and that you’re not reusing nodes in a way that would loop back to the start. This method uses existing nodes only, with no new allocations, and preserves the relative order of both lists.

Allocating new nodes would violate the constraint of not allocating; inserting the head of the second list into the middle would disrupt the proper order and structure of a simple concatenation; replacing the head would lose all elements from the first list.