public String toString(){
        StringBuffer sb = new StringBuffer();
        for( int i=0; i<DIGITS; i++ ){
            for( int j=0; j<DIGITS; j++ ){
                if( board[i][j] > 0 ){
                    sb.append(board[i][j]);
                }else{
                    sb.append(" ");
                }
                sb.append(" ");
            }
            sb.append("\n");
        }
        return sb.toString();
    }