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