public boolean solveProblem(char[][] board){
        for (int i=0;i<9;i++)
            for (int j=0;j<9;j++){
                if (board[i][j]=='.'){
                    for (int t=1;t<=9;t++){
                        board[i][j] = (char)('0'+t);
                        if (isValid(board, i, j)&&solveProblem(board))
                            return true;
                        board[i][j] = '.';
                    }
                    return false;
                }
            }
        return true;
    }