private function initialise()
    {
        for ($i = 0; $i < $this->size; $i++) {
            for ($j = 0; $j < $this->size; $j++) {
                $this->grid[$i][$j] = null;
            }

            $this->rowsAvailable[] = $i;
            $this->columnsAvailable[] = $i;
        }
    }