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Round 2 6th Sudoku


As the competition is over now I wanna ask about the 6th sudoku, the 60 point classic one. Was it only possible to solve it using so-called 50-50 method (randomly trying one of two possibilities) and not logically? I managed to solve first 5 sudokus without problems but in this one I had to use 50-50 twice to get it solved (luckily still managed it on time)...


Why is what you are calling the "50-50 method" not an acceptable method to require in a puzzle? Every step in a logic puzzle such as sudoku is a use of the "50-50 method," since the number you are placing could go in any of up to nine different spots, but all but one of them leads to a dead end. Any techniques that people develop/use for solving puzzles are basically just shortcuts to lessen the effort required to confirm or deny that something goes in a particular spot (often by decreasing the number of steps it takes to reach a dead end). As long as a puzzle has a unique solution though, it can be solved "logically."

Clearly a lot of solvers have an idea of "logically solvable" that is not equivalent to "has a unique solution". See or for some discussion.

That said, while I didn't solve the 60 pointer during the test, I did afterwards, and it didn't feel like "unsolvable", though I can't say I remember the solving path exactly.

It wasn't too difficult... one X-wing and one XY-chain (my preferred method) and its done.

driv4r's picture

Yes, as rob said, by saying it wasn't logically solvable I didn't mean that it didn't have a unique solution, you kinda missed my point or only partly understood me ghirsch.
What I meant by the "50-50 method" is that to fill a particular cell the only way is to guess i.e to try 1 of 2 options (if it's wrong try the other one) and that there is no other logical way to decide, which option (number) is correct.
And I didn't say that it's not an acceptable method, though I do admit that I would prefer not to have those in competitions.

Anyway, as I understood, it was actually possible to solve this one without guessing, so those that did solve it logically, care to explain how?
As I said, I got it solved myself but only thanks to guessing twice, so it would be good to know how to properly solve it...

You can see lot of methods explained here: The difficulty ordering is personal preference; for me it is easier to find and use XY-chain (= Y-wing for 3 cells) and Unique Rectangle rather than X-wing...