@Override
  public void clear() {
    root = null;
    size = 0;
    modCount++;

    // Clear iteration order
    Node<K, V> header = this.header;
    header.next = header.prev = header;
  }
@Test
  public void testClear() {
    LinkedTreeMap<String, String> map = new LinkedTreeMap<>();
    map.put("a", "android");
    map.put("c", "cola");
    map.put("b", "bbq");
    map.clear();
    assertThat(map.keySet()).isEmpty();
    assertThat(map).isEmpty();
  }