- cout << " " << e->origin_vertex->id << " -> " << e->terminal_vertex->id
- << " [style=bold,color=black,label=\"" << -e->length << "\"];" << endl;
+ (*os) << " " << e->origin_vertex->id << " -> " << e->terminal_vertex->id
+ << " [style=bold,color=black,label=\"" << e->length << "\"];" << endl;