An implementation of a brute-force algorithm to solve Sudoku puzzles fails if a cell is discovered in which placing any of the digits 1-9 would be an illegal move.
The implementation is written in C, with the board represented by a 9×9 array. The solver counts down from 9 until a legal number’s reached, and if none can be reached, it outputs a zero in its place.
A zero also represents a cell to be filled in. Here’s the output (truncated) if a string of zeros (an empty board) is the input:
9 8 7 6 5 4 3 2 1
6 5 4 9 8 7 0 0 0
Those last three zeros are there because the values filled in previously aren’t changing. How can I stop the solver from failing like this?
If you would currently put a zero in a spot, instead go back to the previous spot you put a number in and continue to count down till you find another value number for that spot.
For instance, in your example:
Instead of putting the zero in below the three, you would instead go back and try putting a 6 in below the 4.