@Erhu, proving that sudoku has multiple solution may be done in different ways and one of them is not that time-consuming.
Think of what it means that there are several solutions in terms of the resulting elements positions & values.

I run my solution 20 times. I got 4 timeouts, and remaining 16 runs had quite comsistent times of 5-8s. So yes, it seems that its possible to get a pessimistic case, but I think the ratio is not terrible, and i also think there is many better solutions than mine.

All katas have random tests (or should have). You give no proof that tests would be buggy. If you have not been able to solve this kata (I have not either) this cannot be considered a kata issue.

Try to practica on easier ones, 2kyu katas are supposed to be hard coding challenge.

How did others, including me, solved it then?