public Map<V, Integer> outDegree() {
        Map<V, Integer> result = new HashMap<>();
        for (V vertex : neighbors.keySet()) {
            result.put(vertex, neighbors.get(vertex).size());
        }

        return result;
    }
@Test
    void outDegree() {
        Map<Character, Integer> result = graph.outDegree();
        Map<Character, Integer> expected = new HashMap<>(7);
        expected.put('A', 1);
        expected.put('B', 2);
        expected.put('C', 0);
        expected.put('D', 1);
        expected.put('E', 0);
        expected.put('F', 1);
        expected.put('G', 0);
        assertEquals(expected, result);
    }