code: private void swap(int i, int j) {
          int t = pq[i];
          pq[i] = pq[j];
          pq[j] = t;
          qp[pq[i]] = i;
          qp[pq[j]] = j;
      }
docs: Swap i and j items of pq and qp.
  @param i item index
  @param j item index