code: private int findMinGrandChild(Comparator comparator, int index)
    {
      int leftChildIndex = getLeftChildIndex(index);
      if (leftChildIndex < 0) {
        return -1;
      }
      return findMin(comparator, getLeftChildIndex(leftChildIndex), 4);
    }
docs: Returns the minimum grand child or -1 if no grand child exists.