The New Sudoku Players' Forum

[Andrea0095]

Hi guys,

This is my first topic!!
I'm stuck on a "very hard" puzzle found on the internet.
I'm looking for the right technique that can help me solving this puzzle, but I cannot find the right one.

I'm stuck on the 51 step of the solver:
https://github.com/Andrea0095/Andrea/issues/1

This is the starting grid:
https://sigh.github.io/Interactive-Sudo ... 3%7ER9C9_2

What do you suggest?
Thank you in advance

Andrea

[m_b_metcalf]

I suggest you consider the three cells r1c3, r1c4 and r3c2.

Regards,

Mike

[Leren]

Starting Grid.

Code: Select all

*-----------*
|.1.|...|9.7|
|...|42.|...|
|...|.9.|...|
|---+---+---|
|65.|...|...|
|.78|1..|...|
|4.2|...|5.6|
|---+---+---|
|...|3..|..1|
|.6.|7..|8..|
|8.7|5..|3.2|
*-----------*
.1....9.7...42........9....65........781.....4.2...5.6...3....1.6.7..8..8.75..3.2

Don't read on if you don't like spoilers, but some do, so here goes.

Code: Select all

*-------------------------------------------*
| 2   1  b46  |a68  5 3  | 9    468    7    |
| 37 c89  569 | 4   2 17 | 16   3568   358  |
| 37  48  456 | 6-8 9 17 | 1246 234568 3458 |
|-------------+----------+------------------|
| 6   5   1   | 2   3 4  | 7    89     89   |
| 9   7   8   | 1   6 5  | 24   234    34   |
| 4   3   2   | 9   7 8  | 5    1      6    |
|-------------+----------+------------------|
| 5   2   49  | 3   8 69 | 46   7      1    |
| 1   6   3   | 7   4 2  | 8    59     59   |
| 8   49  7   | 5   1 69 | 3    46     2    |
*-------------------------------------------*

If you follow Mike's hint, you will find an XY Wing in the cells marked a-b-c. One of r1c4 or r3c1 must be 8, which solves r3c4 but not the whole puzzle.

Code: Select all

*-------------------------------------------*
| 2  1  b46  | 68 5 3  | 9     a468    7    |
| 37 89  569 | 4  2 17 | 16     3568   358  |
| 37 48  456 | 68 9 17 | 126-4  234568 3458 |
|------------+---------+--------------------|
| 6  5   1   | 2  3 4  | 7      89     89   |
| 9  7   8   | 1  6 5  | 24     234    34   |
| 4  3   2   | 9  7 8  | 5      1      6    |
|------------+---------+--------------------|
| 5  2  c49  | 3  8 69 |d46     7      1    |
| 1  6   3   | 7  4 2  | 8      59     59   |
| 8  49  7   | 5  1 69 | 3      6-4    2    |
*-------------------------------------------*

Alternatively, if you consider the 4 cells a-b-c-d at the same position, it's easy to show that one of a or d must be 4, so you can remove 4's from r3c7 and r9c8, and the puzzle solves with singles from there.

That's called a Skyscraper, and may not be used on some sites. Anyway, if you want to read up on them try here. Hope that helped.

Leren

[Andrea0095]

Thank you so much guys,
I have been understood both the techniques!!