public static Stream<Vertex> reverseDepthFirst(Graph g) {
        return reverseDepthFirst(g.getLeaves());
    }
@Test
    public void testReverseDFSVertex() {
        DepthFirst.reverseDepthFirst(g.getLeaves()).forEach(v -> visitCount.incrementAndGet());
        assertEquals("It should visit each node once", visitCount.get(), 3);
    }