Improve At Sudoku: What To Do When You Get Stuck

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Hi! We're Simon Anthony and Mark Goodliffe, two of the UK's most enthusiastic puzzle solvers. We have both represented the UK at the World Sudoku Championships and the World Puzzle Championships. We're also "cryptic crossword" aficionados. Mark is the eleven-time winner of The Times championship and Simon is the former record holder for most consecutive correct solutions to The Listener crossword. We hope we can help your puzzle solving while also introducing you to some of the world's best puzzles.

Thank you for watching!
Simon and Mark

[davidhuff]

I have a request. Can you pick a relatively hard puzzle and solve it on camera without explanation so we can see how fast you do it without walking us through it at the same time. Then walk through it!

[RealRuler]

Whenever he says 'we've got some cool logic going on', I know I'm either going to learn something or be lost...

[ItsSansom]

That empty rectangle thing is super interesting. Became pretty clear how it worked once you try to put a 7 in the non-pencilmarked box. But it's one thing to understand how a trick works. It's another beast entirely to be able to spot it

[vincentcarrocci]

The Empty Rectangle totally baffled me on the first explanation but I watched it again and understood. That was the key that unlocked the rest of the puzzle for me. I have been working these puzzles until I get stuck and then watch the video for a single clue that pushes me forward and then I continue on my own until I get stuck again. It is a great way to learn and I appreciate the wonderful lessons!

[kevinderoo]

8:20 Plot Twist: He actually got stuck himself and secretly called his friend to help him out.

[RandomBurfness]

"An empty rectangle is just a fancy way of saying a number can only point in two directions." Thank you so much, now I finally understand what an empty rectangle is and now I can start implementing the technique in my solves!

[madsli]

I think an easier way to describe the logic at around 9 minutes is that regardless of where you put the 7 in box 1, the top right corner of box 3 cannot contain a 7.

[Mephistahpheles]

"Empty rectangle" means a 2x2 area in a box can't contain that digit. (Note: in this puzzle, that 2x2 is split down the middle....that's okay.)
The result is a single row and a single column that covers all possibilities for that digit in the box.
ONE of those points to a known limitation (the 57 pair below).
Now, we can determine if ALL possibilities rule out (or guarantee) something else in the puzzle by ONLY checking two (row or column) instead of all 5 (each cell) possibilities.

How to find it:
Work from what you know: the 57 pairs.
Do they eliminate any 5's or 7's from the row? (Very basic and should be automatic as soon as you find the 57 pair.)
Do they make an x-wing? (That's the next thing I look for.) Scan around for other limited 5's or 7's. Do they line up? (Or partly line up....maybe a skyscraper?)
Y-wing? (Is there a 7 paired with, or a 5 paired with, something else?)
And, finally, empty rectangle...look perpendicular (up/down in this example) to the 57 pair to other boxes partly filled...enough to restrict it to 1 row & 1 column for either digit.

That's how I go about it.
And....I kinda suck at it. So, if you have a better idea, I'd be thrilled to hear it!


P.S. Ya, I'm aware of some more difficult stuff (like swordfish) but haven't even tried looking for them, yet.

[icetraydec]

Next up
Improve at Sudoku: What To Do When You Suck

[yaycupcake]

What NOT to do when you get stuck: what I did. I um got stuck and left it open for nearly 2 whole days. LOL Now the timer says over 40 hours. rip

[indigobunting]

First time I understood the use of the "empty rectangle" (even though this "rectangle" was divided into two parts instead of a single block). Nicely taught.

[gregwarren]

Like your contributor, I too have been completely stopped with no idea how to proceed and complete a puzzle. I think what might be very interesting is to use the contributors puzzle starting at the point he became stuck, then validating whether his progress was correct, then proceding to show how to move forward. Explain what to look for in order to move on, or in the event his progress was invalid, show how to recognize being at an unsolvable point due to an earlier error.

[sebrofniloc]

I'm late to the party and have just discovered YouTube sudoku. Strangely soothing, and I think I've learned at least one new trick today.

[gregorymorse]

The rectangle technique here is pretty amazing, as its using higher order logic, basically a first level recursion exclusion search. This puzzle is quite hard and being able to spot such a complexity so quickly is spectacular.

[wrathfeeling]

8:51 here you explain about the rectangle shape. I could follow everything up to that point. So now I have to figure out how to recognize the rectangle shape method. Because it didnt made sense how the 7 could point in two directions, either down or to the side.

[SorcerorNobody]

Definitely need to try and remember that empty rectangle trick. This is the first time I've really followed along with the puzzle myself, and after you showed that, I paused and finished the puzzle. It was marvellous watching the whole thing crumble after using just that one technique! Very satisfying.

[grahamrskelly]

This video shows how much work Simon has made to make this channel succeed. I really really like this channel glad it made it.

[DougCube]

@7:27 You solve a 2 in r5c9 and say "and that might be important... he says and then nothing happens as a result". What you missed is that it creates a bi-value cell of 39 where your cursor is pointing (r5c4), which creates a locked-pair in box 5. As a result, 1 is the only candidate for r5c5 in box 5, but in your actual solve you didn't get it until 6 minutes later (13:25).

[yowkashing]

I had a slightly different approach at the empty rectangle portion. Spotting the possible positions of 7 in row 1 and in row 6 led to an X-wing with a fin leading to elimination of 7 in the third row of the first box. This led me to conclude 7s are exclusively in the first row of box 1, and in row 2 of box 3. The rest followed as per Simon's.

[jarvis]

I was stuck until he pointed out the empty rectangle. That was all I needed.