递归实现

public void dfs2(V begin) {
    Vertex<V, E> beginVertex = vertices.get(begin);
    if (beginVertex == null) return;
    dfs2(beginVertex, new HashSet<>());
  }
  
  private void dfs2(Vertex<V, E> vertex, Set<Vertex<V, E>> visitedVertices) {
    System.out.println(vertex.value);
    visitedVertices.add(vertex);

    for (Edge<V, E> edge : vertex.outEdges) {
      if (visitedVertices.contains(edge.to)) continue;
      dfs2(edge.to, visitedVertices);
    }
  }