What's your approach to solving?

Advanced methods and approaches for solving Sudoku puzzles

What's your approach to solving?

Postby Homer » Sun Jul 10, 2005 10:49 pm

Faced with a hard/Fiendish Su Doku, what strategy do you have to get a complete solution as quickly as possible?

I have the following approach - not necessarily the best, but it is what I use - obviously I would welcome suggestions!

I have a three stage approach, the first stage I call the "Rule of 2", the second I call "All candidates" & the third is the "Series of Techniques". These are my wordings & I am sure they won't mean anything to you without a bit of further explanation. So here goes……..

The Rule of 2 is what I start off with. Starting with the digit "1", I check if there are any cells where a “1” can be securely placed or if there is a 3x3 block where there are only two candidate cells where a "1" could be (hence the Rule of “2”). I then do the same for the other 8 digits, “2” through to “9”. I usually repeat this stage, as even with a "Fiendish" puzzle, the chances are that at least one or two cells will have been solved in the first iteration, which might then generate a few more Rule of 2’s.

Next stage is “All Candidates”. You guessed it, for each 3x3 block, I now fill in all candidate numbers for each cell, taking note of any candidate digits that I have already input.

Finally, I start trying various techniques to either reduce the number of candidates for cells, or indeed, putting a digit into a cell. There are various techniques; typically I first check for any pairs of cells in a group (row, column or 3x3 block) that have only the same two candidates – obviously I can then eliminate any other instances of those candidates in the same group. Looking for X-wings usually comes much later as I find it takes time for me to try to find them in the first place! Every time I complete a cell, I then effectively go back to stage 1 & check if a candidate “1” can be in any cell, followed by “2”, etc. Failing any new cell being completed, I then try another “technique” to reduce candidates.

Using this approach, I have so far solved every Su Doku I have come across (in essence it is the “techniques” I use in stage 3 that really cracks the hard puzzles). However, back to my original question; this is how I go about tackling Su Doku puzzles. How do others go about solving them quickly? How can I speed things up?

Suggestions welcomed!
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Joined: 20 June 2005

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