has_a_cycle $g->has_a_cycle Returns true if the graph has a cycle, false if not. find_a_cycle $g->find_a_cycle Returns a cycle if the graph has one (as a list of vertices), an empty list if no cycle can be found. Note that this just returns the vertices of a cycle: not any particular cycle, just the first one it finds. A repeated call might find the same cycle, or it might find a different one, and you cannot call this repeatedly to find all the cycles.