public interface VertexVisitor<V> {
    boolean visit(V v);
  }

//广度优先搜索测试
  static void testBfs() {
    Graph<Object, Double> graph = directedGraph(Data.BFS_02);
    graph.bfs(0, (Object v) -> {
      System.out.println(v);
      return false;
    });
  }