D  Food for thought

This section is just a bonus for those who are interested in Sudoku. If you finished all of the earlier work, you are done with the final project. Congrats!

Not all puzzles can be solved using the method you've implemented. As an example, consider this puzzle:

If you were to run it in your program, you would get the same puzzle back, with no new big black numbers. But, we actually know one more number.

From the 7s, 8s, and 9s in the top and in the bottom of the puzzle, we know that the center sub-grid's 7, 8, and 9 must be in the center column. We also know that the 8 and the 9 cannot be in the center row, and thus the 8 and 9 in the center sub-grid must be in the starred cells. (We don't know which one goes in which cell, however.) This means the only possible place for the 7 in the center sub-grid is the center of the puzzle.

 

Happy Winter.