Can all directed graphs be topologically ordered? If we add a relation in the description of course relations, so that one needs to study data structures (C5) first before studying introduction to...

Can all directed graphs be topologically ordered?

If we add a relation in the description of course relations, so that one needs to study data structures (C5) first before studying introduction to computer science (C1), would not that be illogical? In that case, we cannot perform topological ordering, since if there is a mutual dependency between C1 and C5, we cannot ascertain the order of the two vertices. In a directed graph, this situation is described as a cycle, as shown in →Fig. 2.58.