Interesting. I don't see one needs the contrivance of swordfish or x-wing. Simple elimination results in.

- Code: Select all

9 2 5 | . 6 . | 1 4 .

. 7 1 | . . . | . 2 .

. 8 3 | 1 . . | . 5 9

-------+-------+------

5 4 . | 6 . 3 | . 8 1

. 6 . | . . . | . 3 .

3 1 . | 8 . . | . 9 6

-------+-------+------

1 3 . | 5 . 9 | . 6 .

. 5 . | . . . | 9 7 .

. 9 . | . 4 . | 3 1 5

At which point we note that either (7,3) or (9,1) is 7.

If (9,1) is 7 then (6,1) is 8

If (7,3) is 7 then (6,6) is 7 and (6,1) is still 8

Hence (6,1) is 8. From here (1,8) is 2 and (4,8) is 3.

The rest falls out with simple elimination.

My technique is not to learn all these stock patterns but to look for a simple switching pair in a 3x3 box that don't share the same row or column. They generally imply some constraint elsewhere. If I see the patterns I'll use them, but I generally don't use anything more complicated than x-wing.

However I would say that the patterns are useful if you're trying to set a puzzle of a desired difficulty.

Richard