How do you reverse a doubly linked list in place?

• A. Traverse the list, swapping the next pointers only.
• B. Traverse the list, swapping the prev pointers only.
• C. Traverse nodes, swap their prev and next pointers; after finishing, set head to the last processed node.
• D. Create a new list by prepending each node.

Answer: (C)

Reversing a doubly linked list in place hinges on flipping the direction of both pointers for every node and then choosing the new head as the last node you processed.

As you walk from the current head through the list, swap each node’s prev and next pointers. After you swap, you should move to the node that used to come after it—this is now reachable via the node’s prev pointer. Keep track of the last node you processed. When you reach the end, that last node becomes the new head. Using this approach, every link is reversed in place and the head is correctly updated to reflect the new start of the list.

If you only swap one pointer, the other pointer still points the wrong way, so the list’s traversal becomes inconsistent. Creating a new list would not be in place and would use extra space. The described method handles both pointers for each node and updates the head to the last processed node, giving a correct in-place reversal.