@Test
public void inDegree() {

    Map<Character, Integer> result = graph.inDegree();
    Map<Character, Integer> expected = new HashMap<>(7);
    expected.put('A', 0);
    expected.put('B', 1);
    expected.put('C', 1);
    expected.put('D', 0);
    expected.put('E', 1);
    expected.put('F', 1);
    expected.put('G', 1);
    assertEquals(expected, result);
}