public boolean isValidSudoku(char[][] board) {
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                if(board[i][j] == '.')
                    continue;
                if (isValidBox(board, i, j))
                    continue;
                return false;
            }
        }
        return true;
    }