Solver App for Android
Strategies for Popular Number Puzzles
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Feedback and Questions

I've received a lot of interesting comments and questions from Sudoku fans over the last few years and this page is where I try to answer them. I'm also directing Str8ts feedback here. Please feel free to drop me a note on the side of the page. Or you can email me directly at .

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Many thanks to all the people who have helped improve the solvers and strategies with their feedback!

Tuesday 29-Aug-2023

... by: Tuner, USA

I ran into a puzzle that I solved using what I call "double Medusa". I don't use colors, it confuses me, I use letters. A/B instead of colors. When A/B does not solve the puzzle, I go to C/D, E/F if necessary. Today I found a puzzle that I solved when A/B and C/D occupied the same cell in two different places indicating that D could not be correct because it occupied cells containing and A and another with a B. All D's are eliminated. Manual testing confirmed.

Is this new? Did I do good?

Andrew Stuart writes:

I think it is possible for two medusa nets to be isolated from one another. And there might be inferences one could deduce from that, especially as not much board space will be left. Reminds me of this

Thursday 24-Aug-2023

... by: aw, Germany

A strange constant: The Daily Sudokus on Wednesday and Thursday are rated as tough - and I can't solve the Wednesday puzzle and have no problem with the Thursday issue.

Any explanation ... ?

Andrew Stuart writes:

No sure. they are from the same pool, should be random within a grade and no particular selection for a day is made if they are the same grade. Perception when no pattern?

Wednesday 2-Aug-2023

... by: Sudhir, India

Wow!! I input an Expert grade puzzle and tool pointed to the critical clue, the way a guide does.

Wednesday 12-Jul-2023

... by: Victor, Brazil

Load Sudoku:

Hi Andrew,

If you please could check why the grader is giving this soduku a 41 ("Gentle/Easy'). I was cracking my head trying to figure why I was taking so much time on a easy sudoku, but I found the solution uses a Y-wing, so it makes sense now.

Love the solver, it helps a lot.

Thank you

Andrew Stuart writes:

Yes this is a very easy puzzle with one bottleneck. Collapses quickly but the y-wing doesn't boost the score enough. I try and filter such puzzles from my players if I can. But useful for the strategy pages if people want to find those strategies

Monday 10-Jul-2023

... by: bruce amos, Canada

Hi Andrew:
Many thanks regarding you site. The sudoku methods are really outstanding.
This comment is regarding the killer sudoku solver which I applied to a hard problem that I got off the net.
The original problem:,121100000814000019000008000013001400160018150024000000180000000000000016000033000000000000001900001718000000180800000000001200000017000703001800001600000013000000

I stepped throught the solver until it ran out of known strategies.
When the solver got stuck we had candidates:
C4:1234568, E3:3456789, E4:3456789
G1:23456789, G2:1234, G3:1234

It is possible to make progress in this position:
Firstly, the points C4, E3, E4 total 19 as the total of all cages that intersect the rightmost five columns exceeds 45*9 by 19. This means that E3~3 since otherwise C4 and E4 would total 16 and have to be 7 and 9. This is impossible since C4 cannot be 7 or 9.

Also, the total of G1,G2,G3 must be 10 since the total of all cages that intersect the bottom two rows is 100. Since G2,G3 are both 1,2,3,4 G1 must be 5,6 or 7.

I'm sure that once the solver handles this type of situation it will noticably improved.

Andrew Stuart writes:

Hi Bruce. Yes I like these explanations. I think the solver can be removed if I can identify those overlaps. Thanks for the example

Sunday 2-Jul-2023

... by: John, Ireland

Dear Andrew,
I'm a huge fan of yours. Thanks for this fascinating website. I would appreciate it if you could clarify two things for me:

1- How the grading system works? Take a look at these three sudokus please:




First one is dubbed Extreme (score: 546), second one is considered Very Hard (score: 592), and third one is again Extreme (score: 687). How is that possible? Generally speaking, which one should I consider the most reliable? The score, or the Extreme/Very Hard/etc. descriptions?

2- This puzzle (010700400200050070000900050009008002003090600700400300070000000060010900008006030) has been dubbed Extreme Grade, Overall Score: 1199, with 57 steps to solve. However, if you remove 3 from F7, it'll still be standard, Extreme puzzle, but the overall score will reduce to 645, with 42 steps to solve. How is that possible? How come removing a clue makes the puzzle easier?

Thanks for your time! Really appreciate it.

Andrew Stuart writes:

Sorry for the late reply

Have you seen this document?

Basically after I decide where the cut-off points are for grades (based on a statistical review of the scores) I do occasionally promote or demote a puzzle if I think it has too many very hard strategies or too few.

I try and avoid publishing ones right on the border as well but might happen

Thursday 29-Jun-2023

... by: Owen Le Blanc, England

I sometimes use the Windoku Solver to evaluate windokus. I have one (from, number J3981) which I suspect has no solution. Windoku solver says "Or it has multiple solutions (check with [Solution Count])." But there doesn't seem to be a "solution count" button on the "windoku solver". I tried "Solve Path", and I had
"Solver by Andrew Stuart, version 2.30, 23-Apr-2023
Clues: 19
Final Score: 0
Grade: Unknown"
It shows 18 steps, but I'm not sure what this means. Does "Final score: 0" mean there is no solution? Or what is the "Solution", which appears to show the numbers that the solver is able to find, plus the original clues, of couse.
-- Owen

Andrew Stuart writes:

Thanks for this feedback and nice to get something about Windoku

I have checked the puzzle with my offline solver which can perform solution counts. Does indeed have one solution and that makes it interesting

The solver when it gives up shouldn't mention the solution count as I can't figure a way to put it on the server alongside the normal Sudoku one. Next update I'll remove that message.

The 18 steps are the steps it can solve, just no final solution and therefore no score

Wednesday 26-Apr-2023

... by: str8ts, Germany

Hi Andrew,
I can´t open the German website any more - there ist alway Error 404.
So i´m missing one str8ts a day (the second one of the classical player) and nearly even worse: I can´t post any solution times or contact my "str8ts-community"
As someone (whose name should not be used) once said: very sad!
I hope the problem can be solves soon.
Greetings from Germany

Andrew Stuart writes:

Hi all. Just to let everyone know that was subject to an automated (scripted) hack through the platform software it used. Daniel is trying to put together a basic site without fancy tools as a substitute and we'll see what we should do afterwards as the best way forward. Thanks for everyone's patience.

Sunday 16-Apr-2023

... by: Harold Silander, Scotland

Many thanks for your Killer Sudoku Solver. I have been doing them for years and soon realised my limitations - can do Saturday Guardian and Observer but maybe require 2 or 3 shots at it. I was really stuck on 9/4/23 Observer puzzle - I had 4 attempts and none worked. Setting it up here was really easy - and you solved it - absolutely brilliant. I now have to work out how I went wrong - and you have given me a great start. HS.

Andrew Stuart writes:

Excellent! Glad it worked, Killer one lags behind the Sudoku

Tuesday 28-Mar-2023

... by: Djebroun, sudoku

The algorithm of the solver displays the first solution it found at a step. In my point of view, it should be interesting to know the other possible solution. Because for example, sometimes i found an AIC and when i compare with the solver, the algorithm found another AIC.
Thanks for this very good site.

Andrew Stuart writes:

Yes this is one of the top requests and I'd really like to make some kind of cycle through alternatives. Makes for a very complicated user interface though
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Thank-you everyone for all your questions and contributions.