In a linked list, which feature enables O(1) insertion or deletion when the position is known?

• A. A predecessor reference
• B. A hash of the node
• C. Directory index
• D. A random access pointer

Answer: (A)

The essential idea is having a reference to the node that comes before the target position. In a linked list, each node points to the next one. If you know the node right before where you want to insert or delete, you can update pointers directly in constant time: for insertion, link the new node between the predecessor and its next node; for deletion, bypass the target by making the predecessor point to the node after the one being removed. This avoids traversing from the head to find the previous node, which would take linear time.

That predecessor reference is what enables O(1) updates when the position is known. The other options don’t provide the necessary direct pointer manipulation to the previous node: a hash of the node, a directory index, or a random access pointer aren’t standard mechanisms to support constant-time insertions or deletions at a known position in a linked list.