for (; k > 1; k--)
  {
    struct bst_node *q = pa[k - 1];

    if (da[k - 1] == 0)
      {
        q->bst_link[0] = n->bst_link[1];
        n->bst_link[1] = q;
      }
    else /* |da[k - 1] == 1| */
      {
        q->bst_link[1] = n->bst_link[0];
        n->bst_link[0] = q;
      }
    pa[k - 2]->bst_link[da[k - 2]] = n;
  }