to.parents.remove(from);
    }

    List<String> visitAll() {
        return visitAll(vertices.values(), new HashMap<>(), new ArrayList<>());
    }

    List<String> findCycle(Vertex vertex) {
        return visitCycle(Collections.singleton(vertex), new HashMap<>(), new LinkedList<>());
    }

    private static List<String> visitAll(

        to.parents.remove(from);
    }

    List<String> visitAll() {
        return visitAll(vertices.values(), new HashMap<>(), new ArrayList<>());
    }

    List<String> findCycle(Vertex vertex) {
        return visitCycle(Collections.singleton(vertex), new HashMap<>(), new LinkedList<>());
    }

    private static List<String> visitAll(

        addEdge(graph1, "b", "c");
        List<String> expected1 = new ArrayList<>();
        expected1.add("c");
        expected1.add("b");
        expected1.add("a");
        List<String> actual1 = graph1.visitAll();
        assertEquals(expected1, actual1);

        Graph graph2 = new Graph();
        graph2.addVertex("a");
        graph2.addVertex("b");
        graph2.addVertex("c");
        addEdge(graph2, "b", "a");