How would you convert a singly linked list into a circular linked list?

• A. Traverse to the last node and set its next to the head; O(n)
• B. Create a circular dummy node and connect the head
• C. Prepend a dummy node and adjust head
• D. Use a tail pointer to join tail to head automatically

Answer: (A)

Making a singly linked list circular means making the last node point back to the first node so traversal can loop endlessly. In a typical singly linked list you don’t have a direct reference to the last node, because the last node is simply the one whose next reference is null. So you start at the head and walk along next pointers until you reach that tail node. Once you’ve found it, you set its next pointer to the head. This single adjustment transforms the chain into a circle.

The operation runs in linear time, O(n), because you may need to visit every node to find the tail, and it uses only constant extra space since you’re just updating one pointer. Alternatives that introduce a dummy node or prepend one don’t actually convert the existing nodes into a circular structure, and relying on a preexisting tail pointer only works if such a pointer is already being maintained.