while (!stack.empty()) {
      Visit& curr = stack.back();
      if (curr.is_exiting) {
        if (node_map.find(curr.n) == node_map.end()) {
          Node* copy = g->CopyNode(curr.n);
          if (curr.n != start_node) {
            for (const Edge* e : curr.n->in_edges()) {
              auto node_iter = node_map.find(e->src());
              if (node_iter == node_map.end()) {