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!

Monday 16-Mar-2009

... by: Felix, California

I was trying the Escargot sample from your page and it seems the solver runs out of strategies. However, your solution counter does show that there is exactly one solution. Is this a known issue?

Andrew Stuart writes:

There is a page about this puzzle here:

Thursday 12-Mar-2009

... by: Oliver B., Germany

Load Sudoku:

Hi Andrew,

I found this Sudoku X in a German magazine called "Sudoku Logisch Spezial" (Issue No. 14 from June 2008).
Some cells of the grid are restricted to even numbers. These are C7, C8, D3, E5, F2, G1, I2 and I7.
The difficulty is rated "easy"... *sigh*
After spending some hours I gave up, tried again some weeks later and so on - until today.
I remembered your web-site and wanted to be shown what I have overlooked and I the end it claimed "Run out of known strategies."

For you are interested in examples that can't be solved on this page I sent this to you. The SolutionCount-Button did not work! ("Not available just yet")

I don't know whether there is a solution derived from any strategies, maybe it is only a misprint, but the given one on the last pages of the magazine is this:


Apart from that I just can say that this homepage is a great benefit when stuck in a puzzle.
Thanks a lot.

Andrew Stuart writes:

Hi Oliver

Thanks for your example. It is a flawed Sudoku X (or I hope you didn't miss a clue or transpose a number). If what you have given me is correct then the publisher has gone a bad job of checking their puzzles. I find 34 solutions for this puzzle, which is why it can't be solved logically.

(If anyone loads this, change the URL to be sudokuX.htm - it loads in the normal sudoku solver)

Sunday 8-Mar-2009

... by: Fran├žois, France

Absolutely fantastic. Must have cost you a lot of work, how generous to share. Many Thanks.

Andrew Stuart writes:

Built up gradually with a lotta suggestions and constructive criticism from many readers - all appreciated :)

Sunday 8-Mar-2009

... by: Doug M, SE PA

Finally, an explanation and example of the X-wing I understand. Thank you.

Andrew Stuart writes:


Friday 20-Feb-2009

... by: Martin, France

wow! this is nice! great job Andrew !

Andrew Stuart writes:

Thanks ! :)

Sunday 15-Feb-2009

... by: Carl, USA - California

Hi Andrew,

I'd like to put in a plug for a feature. I tried using the save and restore feature on your regular solver page "", and the results were nearly what I had expected. Any solved squares in the saved cookie were restored.

My request is to make this feature much much more useful. I actually use your solver as an aid (an online blackboard mostly) to hold the candidates for each cell, as I work on solving "Killer Sudoku" puzzles. I click on each cell of a cage and enter the possible candidates as restricted by the cage values. What I would like your SAVE function to do, is to save all the candidates from the screen to the cookie, and restore all the selected candidates when RE-LOAD is selected. If a sell is "solved", it will only have one candidate, if it is unsolved, it will contain 2 or more candidates. This could be accomplished by using a comma or semicolon separated list instead of just a list of 81 digits/characters. Since my reason for asking is so that I can save the state when I'm working a puzzle, or stuck and want to return later, the major work in getting back t where I was is resetting the candidates. The current RE-LOAD function is useful, but f makes me re-eliminate all the candidates that I've already eliminated in prior work.



Andrew Stuart writes:

Good idea. I've implemented it. Let me know if you find a bug but I've tested it in several browsers. Not rolled out to Sudoku X or Jigsaw yet though.

Wednesday 11-Feb-2009

... by: Peter B, Los Angeles, CA (ex-pat Brit originally from Oxford, UK)

After sporadic attempts to solve Sudoku puzzles in the LA Times I learned some very basic strategies but there's a limit to how accurately you can work using an erasable pen and the original printed puzzle (!).

So I turned to Google and after looking at a few sites I came across yours - and a welcome breath of fresh air :)

Congratulations on the work you've done - this is an excellent tool for newcomers and old hands alike IMHO. I know my skills have certainly benefited greatly from it. More power to your elbow!

Andrew Stuart writes:

Really appreciated, thanks!

Tuesday 10-Feb-2009

... by: Dennis, USA

Andrew, Is a puzzle possible in which the two diagonals (A1 to J9 and A9 to J1) are also unique 1 - 9 numbers? If so, please post it in at least a "hard" format. Thanks

Andrew Stuart writes:

This is perfectly possible but for Sudoku puzzles it will be a coincidence. It is possible to put this constraint in the way they are created but there is no reason to.

However, you should look at Sudoku X where this feature is done by design. I have a solver and some additional strategies for this puzzle, which is quite fun

Saturday 31-Jan-2009

... by: rodnishi, hawaii

your page is an excellent resource for solving difficult sudokus and learning the complex stategies.

i have a question and i don't see it in the strategy explanation. how does the hidden unique rectangle work? when i was rying to solve a puzzle, one of the steps involved the hidden unique rectangle.


Andrew Stuart writes:

I have decided to split out 'Hidden Unique Rectangles' from the 'Unique Rectangles' strategy as I've had many emails about this. There is a strategy page for it here

- but it was not obvious how to find that page. Hopefully this will make it clearer.

All the best with your solving

Friday 30-Jan-2009

... by: Ygor, Netherands

Hi Andrew,

This is a great site! I am wondering what the score is of the most difficult sukoku that your solve can solve? And do you have this sudoku as an example?



Andrew Stuart writes:

The very high scores are above 2000. My sovler/grader plots puzzles in a bell curve but the right hand side exhibits what is known as a 'long tail' - that is some extreme puzzles will go off the scale usually because of a large number of Almost Locked Sets. While the solver includes Trial and Error strategies such as Nisho and Bowmans Bingo I don't like to include those in the grades. I filter these out as unacceptable for publishing but they would be candidates for very high scores. An example is

which I score as 2528 but this won't show up on the web site grader as I exclude Nishio/Bowmans, but you are welcome to discover a better solve solution that doesn't involve them - I'd be very interested to see one.
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Thank-you everyone for all your questions and contributions.