return i * 2 + 1;
    }

    private int getRightChildIndex(int i) {
      return i * 2 + 2;
    }

    private int getParentIndex(int i) {
      return (i - 1) / 2;
    }

    private int getGrandparentIndex(int i) {
      return getParentIndex(getParentIndex(i)); // (i - 3) / 4
    }
  }

  /**
   * Iterates the elements of the queue in no particular order.
   *