I won't write the program for you, but if you begin at the head, follow the links, and wind up back at the head, it's circular. If you run off the end of the world and dragons eat you (usually manifested by a NULL pointer), it's not circular. Tip: a simple pencil-and-paper representation of your problems is quite often effective. If you're not into destroying trees, an Etch-A-Sketch will do.