Wednesday, May 11, 2011

Sudoku Solving Techniques

(click here for rules)

Technique 7 :  


Swordfish is a more complicated version of X-Wing. In most cases, the technique might seem like much work for very little pay, but some puzzles can only be solved with it. So if you want to be a sudoku-solving master, read on! 

Sudoku Solving Technique  Example A

In example A, we've plotted in some candidate cells for the number 3. Now, assume that in column 2, 4, 6 and 8, the only cells that can contain the number 3 are the ones marked in red. You know that each column must contain a 3. 

Sudoku Solving Technique  Example B

I will give you the punch-line before the joke now; look at example B. We can eliminate 3 as candidate in every cell marked in blue. The reason for this is that if we consider the possible placements of the number 3 in the red cells, we get two alternatives: either you must put 3s in the green cells, or in the purple cells, as example C shows. In any case, each of the rows 2, 4, 6 and 8, must contain a 3 in one of the colored cells, so no other cell in those rows can contain a 3. 

Sudoku Solving Technique  Example C

How do you recognize a swordfish pattern? You look for cells with common candidate numbers that can be chained together like in example D. If you start on, say, the top-left red cell. Then you draw a line either vertically or horizontally until you reach another cell containing the same candidate number. Then you repeat this pattern until you return to the original cell. If you reach the original cell, you have a swordfish pattern!

Sudoku Solving Technique Example D

(click here for rules)

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