### Difficult Sudoku

I blogged about Foxtrot and Sudoku a while ago and it garnered a lot of interest. Given that, I thought folks may be interested in some difficult traditional sudoku puzzles I have come across.

The way I actually determine difficulty is from a program I wrote that solves the puzzles. The program implements a number of techniques to solve the puzzles and determines how hard it is based on the techniques that must be used and how quickly it can be solved. I hope to have the solver hosted where you can get to it soon, so stay tuned.

In any case, here is the hardest puzzle I have come across.

 7 9 1 8
 1 7
 4 8 2
 6 7
 5 9 7 3
 7 6
 2 8 6
 9 5
 6 1 2 9

And another difficult one.

 4 5 2 1
 2
 2 9 7 3
 7 3 4
 1 9 5 2
 5 1 3
 7 8 9 1
 7
 6 5 7 4

Give them a try!

wow. the first sure is a sticky one. after the usual walk through, I only have 2 boxes that I know are correct. and they aren't helping any more.

Posted by MikeTLive on December 17, 2006 at 01:41 PM PST #

First one solved: 2 3 7 8 4 1 6 5 9 9 1 6 7 5 2 3 4 8 5 8 4 6 9 3 1 2 7 6 7 3 2 1 5 8 9 4 4 5 8 9 6 7 2 3 1 1 9 2 3 8 4 5 7 6 3 2 9 1 7 8 4 6 5 8 6 5 4 3 9 7 1 2 7 4 1 5 2 6 9 8 3

Posted by Max on December 17, 2006 at 04:27 PM PST #

still working on the first one. I have started to lean into algebra to solve a1+b1..i1=45 a1+a2...a9=45 a1+b1+c1+a2+b2+c2...c3=45 substituting the 38 variables I already know. I need some double secret strategy at this point! I dont want to resort to guessing and backtracking.

Posted by MikeTLive on December 19, 2006 at 08:49 AM PST #

learned a bit more about sudoku. and finally solved it. I built a spreadsheet table to use as my scratch pad. In the end they fell into place so rapidly that I missed and had to backup about 8 steps to find my error. I will to have to revisit this one in a week and see how well I do.

Posted by MikeTLive on December 20, 2006 at 02:11 PM PST #

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