Core algorithm of Sudoku game in Java version (I)

I used JavaScript to write a Sudoku game when learning JavaScript. Recently, I saw some Java content, so I wanted to make a Sudoku game in Java on a whim.

Now I will share all the code and learn from you. Of course, there are all kinds of problems and deficiencies in the code. I hope all friends can criticize and give advice.

The following is the core algorithm for generating Sudoku map. The algorithm is not very good, and some ideas on the Internet have been referred to before:

The complete package can be downloaded on GitHub: https://github.com/houlongchao/s