assertEquals(2, d.getParents().size());
        assertTrue(d.getParents().contains(a));
        assertTrue(d.getParents().contains(c));
    }

    @Test
    public void testCycleDetection() throws Exception {
        Graph graph1 = new Graph();
        addEdge(graph1, "a", "b");
        addEdge(graph1, "b", "c");

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