public List<Integer> getEmptyCellIds() {
        List<Integer> cellList = new ArrayList<>();
        
        for(int i=0;i<BOARD_SIZE;++i) {
            for(int j=0;j<BOARD_SIZE;++j) {
                if(boardArray[i][j]==0) {
                    cellList.add(BOARD_SIZE*i+j);
                }
            }
        }
        
        return cellList;
    }