Sudoku is a puzzle game played on a 9x9 grid, subdivided into 3x3 sub-grids called "regions". The objective of the game is to fill in the blank cells with the numbers 1 through 9, such that each number appears exactely once in each row, column and region.
* For Linux and Mac OS users:
```
$ make
$ ./sudoku
```
* For Windows users:
```
> gcc main.c -o sudoku
> sudoku
```