Dr. Sudoku 1.0

This is a brain-twister. You will observe a playing field with 81 cells (9x9). There are numbers in some of the cells, but most of them are empty. The task is to fill the cells with numbers from 1 to 9. There are about 1000 puzzles of this kind, there is also a special option, which allows the players to mark needed numbers in the cells, which makes the task easier