@Override
  public String toString() {
    StringBuffer s = new StringBuffer("Graph mit " + vertexIndex.size() + " Knoten\n");
    int i = 0;
    for (V v : vertices) {
      if (v != null) {
        s.append(v + ": ");
        for (E e : adjList.get(i)) {
          s.append(e.getVertexB() + " ");
        }
        s.append("\n");
      }
      i++;
    }
    return new String(s);
  }