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 21-May-2013

... by: Chris Williams, Fairford, UK

Easily the best killer solver available. Thank you, Andrew for making it available to us. A solver that just solves the puzzle is useless as we know it'll work; how is the question! This talks you through very nicely and has just got me through Times 3147 that had me stuck for days.

This isn't perfect yet and quite often runs out of ideas, but it's the best out there so well done!

Thanks again,


Andrew Stuart writes:

Cheers Chris, kind words thank you. Killer solver still lags behind Sudoku solver somewhat but I've been given some good hints by various people. I'll keep chipping away at it, you can be sure.

Share and enjoy

Thursday 9-May-2013

... by: Amy Hartner, U.S.A.

Thank you so much! A very evil sudoku plagued me for days until I came to this site and learned the X Wing Strategy. I look forward to using it in the future. I feel like a pro.

Andrew Stuart writes:

Very glad the website is working for you, have fun!

Sunday 28-Apr-2013

... by: PeteTy, usa

28apr2013 killer
was rather hard took me about 45 min by P&P

killer solver
order of operations

row 1+2
5(2) cage with outie 4 next step...
Solver comment:
Killer Hard Combinations
KILLER COMBO (Hard) on B7: cage of size 2 with clue of 5+ can only be 1/2/4/7/9, removing 3/5/6/8

just kind of wondering what easy combinations are
5(2) cage cant have 56789 which is probably a human ez step
a 2 cage with a 4 in one cell seems sort of obvious to me

also i was unchecking all the normal sudoku boxes i could
and noticed the check box for diabolical and extreme
preforms an xor with all the checked boxes
I assumed it would turn them all on or off

Andrew Stuart writes:

I split the combinations strategy into two parts because one is much easier than another - in most cases. The 'easy' is where a cage has only one combination. From that it will be fairly obvious what candidates can be pruned. The 'hard' is where several combinations are possible and its more work to work out what can be removed just from the combinations. The solver does take into account what remains so easy and hard are not fixed. However, if you are half way through a puzzle lots of other eliminations may make a so-called hard relatively easy to work out, combination wise. I was thinking in terms of how a human would approach a puzzle and they, I assume, would want to pick off the easy combination cages first.

The XOR on the groups of strategies is by design but I can't remember why I did it like that : )

Thursday 25-Apr-2013

... by: Dale E. Kloss, Portland, Oregon, USA

I was working a Jigsaw Sudoku and it occurred to me to ask:

For a Jigsaw puzzle maker, how many unique puzzle shapes could be made?

Note that I'm asking about ONLY puzzle SHAPES not where each number goes.

Thanks. Dale Kloss

Andrew Stuart writes:

I do not know, to be honest. That’s a difficult combinatorics problem. For a jigsaw shape pattern to be 'nice' I prefer never to use cages that approximate a row or column, ie more than 7 cells in a line. That removes a large number. To still be Sudoku the shapes shouldn't map onto existing constraints. Reflection, rotation and other symmetries multiply the total number of 'unique' patterns.

Sunday 14-Apr-2013

... by: masonx, usa

This is great , I love to do these puzzles in the newspaper. I don't always have enough time before work and could use just a hint or two without screwing up the whole puzzle. If I have time I will do it alone. Thanks for sharing this , it works great.

Saturday 13-Apr-2013

... by: John, USA

What is the "solve path"?

Andrew Stuart writes:

The series of strategies used to solve the puzzle. From the solver these are "minimally required" meaning that they are the simplest strategies at each step that gets some result. I cannot say that the solver solve path is perfectly optimal since it's down to my choice of the order in which strategies are tested, and in the advanced ones, its difficult to say which are more complex, but a computer is linear, so I have to give it an order in which to search. A truly optimal "solve path" would branch through all strategies and work out the least number of operations to solve as well as taking into account the complexity of each choice, but I can't present that on the website at this stage (too complex, too much CPU) but it’s something I'd like to develop further.

Friday 5-Apr-2013

... by: Geoff, England

Sorry Andrew, still getting the 'wait' state on naked quads

There is a Killer Sudoku board I would like you to look at

Click on this link

Andrew Stuart writes:

Your example puzzle helped me find another string with not enough space. Fixed and your puzzle runs through now.
Thanks for writing in and letting me know!

Saturday 23-Mar-2013

... by: Roman, Australia

Solver 1.91 is hanging up and giving message: Scripts out of date - clear your cache

Andrew Stuart writes:

Hi All
I just figured it out this morning. I’d accidentally deleted a couple of lines of code in the last update – to do with the step-by-step solve part. Odd thing is they are so critical I don’t see how it worked when I tested it. Or how it sometimes works and sometimes don’t. Have a go now.

Friday 22-Mar-2013

... by: ejshan, usa

the grader doesn't seem to be working any longer. use the site almost every day

Andrew Stuart writes:

Are you seeing version 1.91 (the latest) or 1.90 ? I updated all the solvers yesterday and any cached pages will not in all likelyhood work

Thursday 21-Mar-2013

... by: Rich Rice, USA

having problems with naked quads says "wait" forever

Andrew Stuart writes:

That "wait" message means the back end is not returning a result. I updated the solvers yesterday and I suspect it’s a caching issue. Are you seeing version 1.91 (the latest) or 1.90? Windows or Mac?
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Thank-you everyone for all your questions and contributions.