code: public static int[] sort(double[] arr) {
          int[] order = new int[arr.length];
          for (int i = 0; i < order.length; i++) {
              order[i] = i;
          }
          sort(arr, order);
          return order;
      }
docs: Sorts the specified array into ascending numerical order.
  @return the original index of elements after sorting in range [0, n).