Stuck on very hard puzzle from sudoku.cba.si

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Stuck on very hard puzzle from sudoku.cba.si

Postby Andrea0095 » Tue Jun 21, 2022 9:30 am

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
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Re: Stuck on very hard puzzle from sudoku.cba.si

Postby m_b_metcalf » Tue Jun 21, 2022 2:53 pm

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

Regards,

Mike
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Re: Stuck on very hard puzzle from sudoku.cba.si

Postby Leren » Tue Jun 21, 2022 8:44 pm

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
Last edited by Leren on Wed Jun 22, 2022 8:22 pm, edited 1 time in total.
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Re: Stuck on very hard puzzle from sudoku.cba.si

Postby Andrea0095 » Wed Jun 22, 2022 3:10 pm

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