public List<V> getNeighbors(V vertex) {
        return containsVertex(vertex) ? neighbors.get(vertex) : new ArrayList<>();
    }
@Test
    void getNeighbors() {
        List<Character> result = graph.getNeighbors('B');
        List<Character> expected = Arrays.asList('C', 'F');
        assertEquals(expected, result);
    }