What is the standard approach to inserting at the tail when no tail pointer exists?

• A. Traverse to the last node, set last.next = new; new.next = null.
• B. Insert at the head by using head pointer.
• C. Set head.next = new; new.next = null.
• D. Create a new list with the node as the only element.

Answer: (A)

When you only have a head pointer and no tail pointer, adding to the end means you must locate the current last node by walking from the head until you reach a node whose next is null. Once you find that last node, you attach the new node by setting last.next to the new node and then set the new node’s next to null. This preserves the existing list and places the new element at the tail. Because you have to traverse from the head, this operation runs in linear time with respect to the list length.

The other approaches don’t correctly perform tail insertion in this scenario: inserting at the head moves the front of the list to the new node rather than extending the end; setting the next pointer of the head directly can break the list structure or misplace the new node, especially if the list has multiple elements or is empty; creating a new list with only the new node discards all existing nodes.