Sudoku Players' Forum (clone)

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59...4....16.8.....8.1....53.....14..654.278..48.....99....3.5.....4.23....8...97

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*------------------------------------------------*
| 5  9   37-2 | 2367  367-2 4    | 368  1    268 |
|a27 1   6    | 359   8     59   | 39  a27   4   |
| 4  8  b237  | 1    b2367  679  | 369  67-2 5   |
|-------------+------------------+---------------|
| 3  27  9    | 567   567   8    | 1    4    26  |
| 1  6   5    | 4     9     2    | 7    8    3   |
|a27 4   8    | 367   1367  167  | 5   a26   9   |
|-------------+------------------+---------------|
| 9  27  4    | 267   167-2 3    | 68   5    168 |
| 8  5   17   | 679   4     1679 | 2    3    16  |
| 6  3  b12   | 8    b125   15   | 4    9    7   |
*------------------------------------------------*

XWing in 2's r26 c18 => - 2 r3c8; Then XWing in 2's r39 c35 => - 2 r1c35, r7c5

You've missed two X Wings on 2, one following on from the first, that removes some 2's as marked. This doesn't solve the puzzle but you should not be overlooking X Wings on puzzles from commercial sites, as these may be the crucial moves.

Nevertheless something a bit more is needed to solve this puzzle. I've ignored the X Wings and used an XY chain of length 4. This is like an XY wing, that has 4 cells in it instead of 3.

Code: Select all

*---------------------------------------------*
| 5   9    37  | 2367 367  4    | 368 1   268 |
| 27  1    6   | 359  8    59   | 39  27  4   |
| 4   8    237 | 1    2367 679  | 369 67  5   |
|--------------+----------------+-------------|
| 3  a27   9   | 567  567  8    | 1   4  b26  |
| 1   6    5   | 4    9    2    | 7   8   3   |
| 27  4    8   | 367  1367 167  | 5   26  9   |
|--------------+----------------+-------------|
| 9   2-7  4   | 267  167  3    | 68  5   168 |
| 8   5   d17  | 679  4    1679 | 2   3  c16  |
| 6   3    12  | 8    125  15   | 4   9   7   |
*---------------------------------------------*

XY Chain Length 4 in Cells a-b-c-d => - 7 r7c2, which solves the puzzle.

XY Chains are discussed here.

Leren

I'm a bit confused about these X wings.

I didn't think the first one (a) would be possible because of the 2 in r3c8?

UPDATE: HuDoku page on X Wings explains this!

And with the XY Chain:

If (a) is 2, then r7c2 must be 7. But what happens if (a) is 7? Is this not possible?

I understood the first XY Chain example on the HuDoku page lol