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2davidc8's avatar

What is your best Sudoku strategy?

Asked by 2davidc8 (4135 points ) October 4th, 2012

I tend to go at it randomly. There must be a better way that’ll solve the puzzle faster. If you consider yourself very good at it, could you explain your strategy to me?

(Yes, I know that there was a similar question asked recently, but no real strategies were given in the answers.)

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9 Answers

rooeytoo's avatar

I approach it methodically. First I look at the 3 horizontal boxes and see if the duplicate numbers lead me to find where that number belongs in the third box. Then I do that with the 2 rows below. Then I do the same thing with the vertical rows. Then I look and see if there are any rows or boxes with 4 or less blanks and see if I can find those numbers. Then I go back to the first box and go through it to see if I can determine the missing numbers and continue to go through each and every box that way. When that is all done then I go back and sort of start over. I put in the little numbers for any row or box with 4 or less blanks. If there are more than that it just gets too cumbersome. I love sudoku, I play it on the computer and iphone frequently.

I often think there must be a better way. I hope some others weigh in with their methods.

filmfann's avatar

At the start there are usually 3 or 4 obvious entries.
Next, I examine any box, line, or column that has 5 or more filled cells. I write in the remaining possible numbers, and use the process of elimination.
The above strategy will solve the 1 or 2 star puzzles. For the more difficult puzzles, I find a cell that has 2 options, and work out how the puzzle would solve in either case.

El_Cadejo's avatar

I do essentially what @filmfann said. It may take me a bit longer to get the numbers in on the harder ones but they’re usually right the first time around so I dont have a 75% completed puzzle and realize I messed up and have to do it all over again.

downtide's avatar

It’s all methodical and there should be no guesswork involved.

First I do all the obvious singles, where there’s only one cell a number can be in. There are usually at least two or three of those, or more in easier puzzles. The early numbers you fill in often lead to more singles.

Next, I look at rows, columns or boxes with 4 or fewer empty cells and I write in pencil at the top of the cell, all the possible options. Look for pairs: if you have two cells with a pair of candidates you can eliminate those two candidates from all the other cells in that row/column/box. You can do the same with triples, and even quads in a more complex puzzle.

At this stage you should fill in the candidates for all the remaining empty cells.

Look for locked candidates next. This means that if the only occurrences of a particular candidate is within the same row/column AND the same box, then you can eliminate that number from the rest of the box.

These three strategies should solve most puzzles except the most difficult; in those cases you’re going to need to start looking for things like X-wings and Y-wings, which is far too complicated to explain here without diagrams. There are plenty of strategy guides on the internet which explain them.

wds2's avatar

I generally fill in the obvious ones and then take a methodical approach, going by number rather than position. I then basically do what @downfide says, but I go through all the possibilities by the number I’m looking for. This allows me to determine when I have finished, say, the fives.

rojo's avatar

Methodically,
first numerically, all the 1’s, all the 2’s etc. be sure and put the possibles into the squares that do not give you a definite answer.
Then the rows, boxes or columns with only two, then only three, then only four. Don’t bother with only 5 except as a last resort.
Oh, and be sure to put the possible numbers in the same place in every box: i.e. the 1 in the upper left hand corner, the 2 next to it, the 3 in the upper right hand corner and so on to the 9 in the lower right hand corner.
I think this is similar to @downtide

Sunny2's avatar

I haven’t read the full question nor any of the answers. I decided to use sudoku to exercise my aging brain. So I never had any instructions and I have never related well to numbers. In the 4 years since I started, I’ve gone from writing all the possibilities in each square (I had to find as large puzzles as I could) to completing 5 6 star puzzles. I just looked at the puzzles and let my brain tell me what to do next. I still have things to learn, because I still can miss the easiest puzzles. Some day my brain will let me know a more efficient way to start the puzzle. There’s something I don’t know yet. Probably a number of things.

Pandora's avatar

I start out with the numbers that repeat the most first and then go to the lines that have the most numbers in it.
If it is the really hard one then I will concentrate on the box that has the most numbers.

2davidc8's avatar

Thanks to all for responding! I think I will start with @downtide‘s strategy and go from there. I may try a combination of strategies if I get stuck!

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