How do you insert a new node after a given node in a doubly linked list?

• A. new.prev = node; new.next = node.next; node.next = new;
• B. new.prev = node; new.next = node.next; if node.next != null: node.next.prev = new; node.next = new;
• C. new.prev = null; new.next = node; node.prev = new;
• D. new.prev = node; new.next = null; node.next = new;

Answer: (B)

Inserting after a given node in a doubly linked list requires updating both directions of the surrounding links so the chain stays connected in both directions. To insert the new node after the specified node, first make the new node point back to the given node and forward to the original next node: new.prev should be the given node, and new.next should be the original node.next. Then link the given node to the new node by setting node.next to the new node. Finally, if there was a node after the given one (i.e., the original next wasn’t null), update that next node’s prev to point back to the new node. This last step preserves the backward link, ensuring the list remains consistent in both directions, whether you’re inserting in the middle or at the tail.

That’s why the correct approach sets new.prev = node and new.next = node.next, updates node.next to new, and, only if there is a node after, updates node.next.prev to new. This handles both middle insertions and appending after the last node cleanly.

The other options fail because they either don’t update the neighboring backward link when there is a next node, attempt to insert before the given node, or unconditionally set the new node’s next to null, which breaks the chain if there is a following node.