Valid Sudoku

36. Valid Sudoku

Determine if a Sudoku is valid, according to: Sudoku Puzzles - The Rules.

The Sudoku board could be partially filled, where empty cells are filled with the character '.'.

Note:
A valid Sudoku board (partially filled) is not necessarily solvable. Only the filled cells need to be validated.

public class Solution {
    public boolean isValidSudoku(char[][] board) {
        if (board == null) {
            return false;
        }

        boolean[] rowMask;
        boolean[] columnMask;
        boolean[] areaMask;

        for (int i = 0; i < board.length; i++) {
            rowMask = new boolean[9];
            columnMask = new boolean[9];

            for (int j = 0; j < board[i].length; j++) {
                if (board[i][j] != '.') {
                    if (rowMask[(int)(board[i][j] - '1')] == true) {
                        return false;
                    } else {
                        rowMask[(int)(board[i][j] - '1')] = true;
                    }
                }

                if (board[j][i] != '.') {
                     if (columnMask[(int)(board[j][i] - '1')] == true) {
                        return false;
                    } else {
                        columnMask[(int)(board[j][i] - '1')] = true;
                    }
                }
                

                if (i % 3 == 0 && j % 3 == 0) {
                    areaMask = new boolean[9];

                    for (int m = i; m < i + 3; m++) {
                        for (int n = j; n <  j + 3; n++) {
                            if (board[m][n] != '.') {
                                if (areaMask[(int)(board[m][n] - '1')] == true) {
                                    return false;
                                } else {
                                    areaMask[(int)(board[m][n] - '1')] = true;
                                }
                            }
                        }
                    }

                }
            }
        }

        return true;
    }
}

Hope this helps,
Michael