sudoku solver

public void (char[][] board) {
    if (board == null || board.length != 9 || board[0].length != 9)
        return;
    dfs(board);
}

public boolean dfs(char[][]board) {
    for (int i = 0; i < board.length; ++i) {
        for (int j = 0; j < board[0].length; ++j) {
            if (board[i][j] == '.') {
                for (char c = '1'; c <= '9'; ++c) {
                    if (valid(board, i, j, c)) {
                        board[i][j] = c;
                        if (dfs(board)) {
                            return true;
                        } else {
                            
                            // the previous level
                            board[i][j] = '.';
                        }
                    }
                }
                return false;
            }
        }
    }
    return true;
}

public boolean valid(char[][]board, int i, int j, char target) {
    // check row and column
    for (int index = 0; index < 9; ++index) {
        char c = board[i][index];
        if (c == target)
            return false;
        c = board[index][j];
        if (c == target)
            return false;
    }
    int row = (i / 3) * 3, column = (j / 3) * 3;
    for (i = row; i < row + 3; ++i) {
        for (j = column; j < column + 3; ++j) {
            if (board[i][j] == target) {
                return false;
            }
        }
    }
    return true;
}