public void addEdge(V from, V to) {
        addVertex(from);
        addVertex(to);
        neighbors.get(from).add(to);
    }
@Test
    void addEdge() {
        graph.addEdge('B', 'G');
        List<Character> result = graph.getNeighbors('B');
        List<Character> expected = Arrays.asList('C', 'F', 'G');
        assertEquals(expected, result);
    }