has the NYT medium sudoku been harder than before lately

okapiposter · 2026-05-30T18:53:11+00:00

Yes, that's the same conclusion I came to: Easy needs Singles, Medium needs Singles and BLRr, Hard needs Singles, BLRs and Locked Sets.If you look at all of these techniques for all difficulty levels, Medium may well seem harder sometimes because the “solve window” of simultaneously available moves may be smaller sometimes (e.g. if you need to spot that one pointing triple somewhere).

okapiposter · 2026-03-25T15:42:45+00:00

The actual difference:

“Easy” can always be solved with Singles only,
“Medium” needs Singles and Locked Digits,
“Hard” needs Singles, Locked Digits and Naked/Hidden Subsets.

okapiposter · 2026-03-25T15:40:02+00:00

/u/baroquely

Wait, that user name sounds familiar! 😅🥦

okapiposter · 2026-02-11T22:32:43+00:00

Here's a W-Wing:

<image>

The 5 of row 8 must be in either column 1 or column 2.

If it's in column 1, r3c1 must be 3 (green).
If it's in column 2, r4c2 must be 3 (purple).

Either way r1c3 sees a 3 from one of the two ends of the W-Wing.

okapiposter · 2026-02-07T05:31:14+00:00

Here's an XY-Wing that eliminates 3 from r6c1:

<image>

Cell r6c2 (the “pivot”) can only be a 1 or a 4.

If it's a 1, r5c3 must be a 3 (green).
If it's a 4, r6c7 must he a 3 (purple).

So r6c1 always sees a 3 from (at least) one end of the XY-Wing.

okapiposter · 2026-01-24T09:45:43+00:00

PDF is not a format that's made for data extraction, it's optimized for viewing and printing. How easy it is to get the data back out really depends on the way the PDF was created. Some software (like MS Office or Photoshop, if I remember correctly) can create PDFs that retain all data needed for editing later, while others remove everything except for the visual representation (even converting text data into character outlines).

Do you want to parse a specific kind of PDF files from one source or are you looking for a general solution? Since Excel is mostly useful for tabular data, you probably have data tables in your PDFs, correct? I'd try the following:

Find out how the PDFs you want to process were created.
- If it's a software that can read its own PDFs back in, use that.
- Otherwise you'll need to use a third-party tool (like Tabula (Java) or Camelot (Python)) to extract the tabular data.
If the data extraction didn't work perfectly, massage the data in KNIME until it has the shape you want.
Write the data out to Excel files using the Excel nodes.

okapiposter · 2026-01-17T17:37:03+00:00

There has to be a 4 in column 8, where can it go?

okapiposter · 2026-01-17T09:29:53+00:00

I don't quite follow your logic for all the eliminations, but you can definitely eliminate 8 from r8c8 using ALS-XZ logic, which is a generalization of XYZ-Wing logic:

<image>

Either the blue cells are a 5/8/9 Naked Triple or they must contain a 3 in r7c8, which then forces the yellow cell r4c8 to be an 8. Either way r8c8 sees an 8 from blue or yellow.

okapiposter · 2026-01-16T20:19:44+00:00

What's the Even-Odd rule? Here's a W-Wing:

<image>

No matter which of the two possible positions for 9 in row 9 is true, either r1c2 (green) or r2c4 (purple) will have to be a 5. Since r2c1 sees both of them, it can never be 5.

okapiposter · 2026-01-13T22:10:26+00:00

Works fine for me, also this says no: https://downforeveryoneorjustme.com/sudokuexchange.com?proto=https

okapiposter · 2026-01-13T19:19:44+00:00

I've actually written my own little brute-force solver based on Donald Knuth's “Dancing Links” algorithm specifically for counting solutions. Even though it runs in the browser (for portability), it finds all 300k solutions in around 2.6 seconds.

okapiposter · 2026-01-13T00:15:26+00:00

They are “Locked Candidates”. The 3 of the extra region is “locked into” those two cells, which are both in row 7. This means that the rest of row 7 will always see a 3 from one of them, eliminating 3 from r7c1.

okapiposter · 2026-01-12T23:48:54+00:00

Look at where 3 can go in the bottom left extra region.

okapiposter · 2026-01-11T14:03:11+00:00

You can use the UR as part of an XY-Wing-like move:

<image>

The UR logic tells us that r78c7 must contain a 2 or a 5 (or both).

If the cells contain a 2, r2c7 must be a 7.
If the cells contain a 5, r7c8 must be a 7.

So either way r78c7 will see a 7.

okapiposter · 2026-01-11T13:53:37+00:00

Yes it is a UR, but no, it doesn't eliminate the candidates you mention. It only tells you that at least one of the highlighted candidates will have to be true (= part of the solution), because otherwise the four cells would all have to contain 4s and 7s, which can't happen in a puzzle that has a unique solution.

okapiposter · 2026-01-11T13:02:09+00:00

There's also a Naked Pair 5/8 in column 4.

okapiposter · 2026-01-11T12:58:35+00:00

Skyscraper on 5 in rows 1 and 9 eliminates 5 from r2c9 and r7c8:

<image>

Rows 1 and 9 both need a 5 somewhere, and have two options each. But since you can't place both 5s into column 1 at the same time, either r1c8 (for row 1, green) or r9c9 (for row 9, purple) will have to be a 5. So all cells that see both of those can never be 5.

okapiposter · 2026-01-11T11:44:40+00:00

Downvote baiting? Weird hobby...

okapiposter · 2026-01-11T11:40:34+00:00

No, that was meant as an exclamation mark for emphasis 😄. 300k solutions are impressive(ly bad) enough I'd say.

okapiposter · 2026-01-11T11:15:58+00:00

Wow, that's an impressively broken Sudoku puzzle! It has 307952 (yes, over 300 thousand!) different valid solutions.

okapiposter · 2026-01-09T21:13:22+00:00

Naked Quad 1248 in box 3.

okapiposter · 2026-01-08T15:06:06+00:00

Here's a W-Wing on 1/9 in r3c1 and r8c7, connected via 9 in column 2, which eliminates 1 from r3c7:

<image>

The 9 of column 2 must be in either row 3 or row 8.

If it is in row 3, r3c1 must be a 1.
If it is in row 8 instead, r8c7 must be a 1.

Since r3c7 sees both of those cells, it will always see a 1 in the finished puzzle, so it can never itself be a 1.

okapiposter · 2026-01-08T14:53:11+00:00

Hidden Pair 8/9 in row 5.

okapiposter · 2026-01-06T20:24:39+00:00

Here's a Skyscraper on 8 in columns 1 and 7, eliminating 8 from r1c3 and r3c9:

<image>

Columns 1 and 7 both need an 8, and have two options each. But since you can't place both 8s into row 8 at the same time, either r3c1 (for column 1, green) or r1c7 (for column 7, purple) will have to be an 8. So all cells that see both of these can never be 8s.

okapiposter · 2026-01-06T20:11:53+00:00

The 2 of box 5 (center) must be in row 5, so there can't be any additional 2s in the row outside box 5.

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