AI Sudoku

public class Sudoku {

    public static final int n = 3;
    public static final int n2 = n*n;

    private byte [][] board;

    public Sudoku()
    {
        board = new byte[n2][n2];
        for(int i = 0; i <n2; i++)
        {
            for(int j = 0; j <n2; j++)
            {
                board[i][j] = 0;
            }
        }
    }

    public String toString()
    {
        String result = "";
        for(int i = 0; i <n2; i++)
        {
            for(int j = 0; j <n2; j++)
            {
                result += board[i][j] + ",";
            }
            result += "\n";
        }

        return result;
    }

    public void fromString(String txt)
    {
        int k = 0;
        for(int i = 0; i <n2; i++)
        {
            for(int j = 0; j <n2; j++, k++)
            {
                board[i][j] = Byte.valueOf(txt.substring(k,k+1));
            }
        }

    }

    private static boolean isGroupLeegal(Byte group[])
    {

        boolean [] visited = new boolean[n2];
        for(int i = 0; i < n2; i++)
        {
            visited[i] = false;
        }

        for(int i = 0; i < group.length; i++)
        {
            if(group[i] == 0)
            {
                continue;
            }
            if(visited[group[i]-1]) {
                return false;
            }
            visited[group[i]-1] = true;
        }

        return true;
    }


    public boolean isBoardCorrect()
    {
        Byte[] group = new Byte[n2];
        //wiersze
        for(int i = 0; i < n2; i++)
        {
            for(int j = 0; j < n2; j++)
            {
                group[j]=board [i][j];
            }
            if(!isGroupLeegal(group))
            {
                return false;
            }
        }

        //kolumny

        for(int i = 0; i < n2; i++)
        {
            for(int j = 0; j < n2; j++)
            {
                group[j]=board [j][i];
            }
            if(!isGroupLeegal(group))
            {
                return false;
            }
        }


        //kwadrat

        return true;
    }

    public static void main(String[] args)
    {

        String sudokuValue = "003020600900305001001806400008102900700000008006708200002609500800203009005010300";
        Sudoku s = new  Sudoku();
        s.fromString(sudokuValue);
        System.out.println(s);
        System.out.println(s.isBoardCorrect());
    }

}