What is the difference between NULL termination and a cycle in a linked list, and how would you test for each?

• A. NULL termination ends when the last node's next pointer is NULL; a cycle loops back to a previously seen node; test for a cycle with Floyd’s cycle detection.
• B. A cycle ends when a tail node's next pointer is NULL.
• C. You can always test for cycles by checking for a NULL pointer.
• D. NULL termination and cycles indicate the same structure.

Answer: (A)

The main idea here is the way a singly linked list ends: either with a real end (NULL termination) or with a loop (a cycle) where a next pointer points back to an earlier node. For NULL termination, the traversal stops when you reach a node whose next is NULL, so you can confirm termination by continuing until you hit that NULL. If you ever encounter a situation where you never reach NULL because you’ve entered a loop, that indicates a cycle rather than a proper end.

To test for a cycle specifically, a classic and efficient method is Floyd’s cycle detection. You keep two pointers moving through the list at different speeds: the slow pointer advances one node per step, the fast pointer advances two nodes per step. If there’s a cycle, the two pointers will eventually land on the same node. If the list terminates, the fast pointer will reach NULL (or its next will be NULL) before they meet, confirming there is no cycle.

In practice, you can also detect cycles by keeping a record of visited nodes, but that uses extra space. Floyd’s method achieves cycle detection in constant space with linear time. So the difference is that NULL termination means there’s a definite end point, while a cycle means the traversal loops back to a previously seen node; testing for NULL termination is a straightforward traversal to NULL, and testing for a cycle is efficiently done with Floyd’s two-pointer technique.