i think this can work... one way to find a topsort is first remove a vertex with 0 indegree and proceed like that until i can't remove any vertex... we are going to proceed in this way but the candidates nodes to remove are going to be in a heap (or set) of pairs... the pair are going to contains (current indeg of the node, node itself). at the beginning put in the set all vertex with indegree 0, when we remove a vertex, update the indeg of all vertex neighbors to him, and proceed removing vertex until all are processed.

You can check your implementation/idea with this problem btw: https://jutge.org/problems/P14952_en

Also, my solution was the same that jcg described above.