Why is a dummy head often used when reversing a list in groups of k nodes?

• A. To ensure the list avoids collapsing when you reverse the last block.
• B. To reserve extra space for upcoming reversals.
• C. To simplify connecting reversed blocks by providing a stable starting point.
• D. To prevent memory leaks during reversal.

Answer: (C)

Using a dummy head (a sentinel node) provides a stable starting point before the real head of the list, which makes reconnecting blocks much simpler when reversing in groups of k nodes. When you reverse a block, you need to attach the new head of that reversed block to the part of the list that came before it. If there’s no node before the head, the first block would require special-case handling. The dummy head gives you that pre-block anchor everywhere, so you can treat the first block the same as any other block and connect its head to the previous portion through the dummy. After each reversal, you can continue by linking the tail of the reversed block to the next block’s head, all using the same logic. At the end, the new list head is dummy.next, so you return the correct head without extra branching or tracking.

This tool doesn’t reserve extra space, prevent memory leaks, or directly affect the last block’s behavior beyond simplifying the reconnection logic. Its value is in providing a uniform, stable point to attach reversed blocks.