A Great Technique in a Classic Sudoku
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- เผยแพร่เมื่อ 10 ก.ย. 2024
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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 twelve-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
This is -almost- my first time on this channel, without any knowledge or practice on sudoku other than the average. And I must say I'm both overwhelmed and excited, this could be a new hobby for me or a source of frustration lol. I'm glad to see these kinds of projects on youtube, nonetheless. Greetings from México!
Welcome to the club. You will get both satisfactions and furstrations, but believe me, its' worthy.
Hello to Mexico from Europe! Enjoy!
Welcome.
That was me not so long ago. Lots of fun awaits you. Welcome!
The way I'd describe what you found is a skyscraper on 4s. A skyscraper is like an "offset" x-wing, where two rows (or cols) have a digit limited to two places, and one of them lines up like an x-wing, but the other does not. Even without spotting that those two cells are clones, that formation proves that 4 had to be in at least one of r3c1 or r7c2. This eliminates 4s from everywhere else it could be in boxes 1 and 7, giving the same result of setting both those cells to 4.
Or a pincer and pivot to rule 4 out of r7c9, Simon uses this from time to time
Yep. Skyscraper is what he found, can also be called a turbot fish. It was neat for him to solve it the way he did. I found myself screaming at R2C4 to be a 3 until he found it.
Same result for me by using empty rectangle on 4s in box 3
thanks. tried to figure out what i used. i did it in Reverse in my mind, not finding the 4 but eliminating it from R1c2 as that forced it in r7c9 making box 3 impossible. from then on puzzle just flows.
@@deejawng thank you. Empty rectangle s on 4 to give a naked single 9 on r3c9
Another way to go about this is using an empty rectangle. Either r3c1 is the 4 or r8c1 is the 4 in column 1. If it's in r8, r1c2 becomes 4, then in box 3, r3c9 becomes 4 and the 4 gets pincered out of box 9. Was very happy to spot that! :)
Yep, that was my solution too. I also got the 1 in R4C4 fairly early on, though I can't remember how!
That poor 3 in box 2; it kept waiting and waiting...
And the naked single 1 after that in box 5...
MORE OF THESE. I would never have spotted that on my own. But once I did, I finished the puzzle in like 2 minutes.
Mark 12:59: (Lopsided x-wing on 4s) "That's beautiful! - I mean - that's what machines can do sometimes - like infinite monkeys ..."😁😏
The 3 was sitting in box 2 for more than half the video.
The 3 in box 1 was a naked single from the start!
He kept marking possibilities around the grid and ignored box 2 which had the most information
which would have helped restrict the 5 to the middle box, and a naked single one. but he found it in the end
they are a bunch of idiots. all of them. they just like to ear themselves.
The 3 in row 2 is what had me yelling today. I also tried solving on a desktop and now I see how much that screws up finding things. Bleah
How so? I'm always keen to improve the software!
@@killroy42 well it was my first try, so even the screen size was an issue. I also need to learn the keyboard command, which will help. Is there a tutorial?
@@rabidsamfan There's no tutorial exactly, but the shortcuts are listed on app.crackingthecryptic.com/
Most importantly:
: "Digit" mode ( on German keyboard layout)
: "Corner" mode
: "Centre" mode
: "Colour" mode
+ : Enter digit in "centre" mode
+ : Enter digit in "corner" mode
+ + : Enter digit in "colour" mode
@@calebspringer1192 Thanks. I will try it. Any chance you can rig it so I can choose a digit and pop it into cells instead of having to select cells and then choose which digit?
@@rabidsamfan Not to my knowledge. They set it up for selection first, then choosing how to fill second.
74:13 - well, I am at the lower end of the skill-spectrum. Took a very long while on a tangent for 1's that let me cross them off of 1 cell which did nothing. After finding nothing for ages, I eventually filled the entire grid with candidates. Never plan on doing that, but after going from row to row, block to block and try to find anything it always happens in the end. Found your 13:00 breaker eventually, but with a "What if R1C2 is a four?-chain, that eventually breaks in Column9."
Felt like a stumble through, rather than a solve. Will keep on going. Eventually I'm gonna get better.
@15:37 There actually was a naked single in r4c4. 2 above, 3 above/in box, 4 below/in box, 5 left , 6 left/in box, 7 right, 8 left, 9 in box. I only mention it because you said, "it wasn't a naked single," @15:52
The 35 pair in column 2 contained the only marked 3 in that entire column during the extensive scanning.. Interesting that it went unnoticed. It was there "by sudoku" even. Good to see that even the pros can fail at scanning on rare occations. :D
The technique I used to get the final deduction with 4s was a 2-string kite- 4s in c1 were restricted to r3 and r8, and 4s in r7 were restricted to c2 and c9- this causes 4 to be eliminated from r3c9.
I used the 4 as a break in too - but used the empty rectangle in box 3 with 4 either in c9 or r1. A 4 in c9 of box 3 and/or a 4 in r1 of box 3 force a 4 in r2c7
Great puzzle!
I used Empty Rectangle on 4s in box 3, which removed 4 as an option in R7C9, leaving a 28 pair in row7. From there it pretty much unfolded in the same way as Mark's solve. I tend to find empty rectangles easier to work with than spotting X-wings/skyscrapers etc.
12:25 that's brilliant! I spent almost an hour trying to find that before I watched your video. Nice job!
ALWAYS happy to see a classic Sudoku featured. I am also always impressed at Mark's ability to intuit patterns quickly. Often without knowing the name of a technique, he somehow sees the pattern. Amazing!
I was able to spot 3 on Column 4, Row 3. And that made me proud lol.
@12:55 I'm happy to see there was a logical way to progress. I had ended up in roughly the same state with the puzzle and could not find a way to logically progress without plugging a guess in, and I'd rather close the puzzle than guess.
The 3 is placeable in box 2, and and x-wing in 5s gets rid of a penciled 5. (You showed it as a possible swordfish.)
I did notice early that the two cells had to be the same, but I didn't see the consequence.
EDIT: 11:30 Place the 3 in the middle 35 of box 2, and the position becomes an X-wing. (The 3 can't go anywhere else in box 2.)
My first try at solving a CtC puzzle before watching the video. I got stuck because I didn't spot the 2-4-5 triple at c2. Got that from Mark and I spotted the X-wing-Skyscraper-whatsamacallit pretty fast on my own and was so proud!
I have no idea what my time is, as I had to work this puzzle over the course of two days. I got stuck very badly and wound up breaking it by discovering an empty rectangle on 4s in Box Three. It produced a naked single 4 in Box Seven and, because I had two X-Wings on 5s that said 4 occupied a cell of, I was able to fill in the 5s immediately and the puzzle unwound from there.
LOL, I got the 5-7pair in box 5 early on because the 3 in box 2 was clear long before he got it.. marked even the two 45 cells that cracked it open, but failed to act correctly on it... lesson learned! thanks!
That 4 offset x-wing had me fooled for a long time. Finally found it and unravelled the puzzle. Agreed. Computers and infinite monkeys. Good puzzle. :)
36 minutes. I spotted an empty rectangle on 4s to get over the hump myself. It cleared up box 3 and the top chute quite nicely.
it all came down to this weird 45 pair.. i tried to solve this by myself (im a beginner) and i couldnt so i watched the video and as soon as i saw the 45 resolution i went back and finished the pazzle. really interesting but i would NEVER think of that on my own.
34:36 but didnt saw the single 3 and the thing with the 4's, this channel is improving me a lot :)
I love it when Mark solves classic sudokus ❤️
18:57 for me, The puzzle felt somewhat easy except for a bit i found on r2c9 not being able to be a 9 because it eliminates all options for 4 from box 7.
7:57 how did he place that 7. I'm a beginner I didn't get it
14:30 for me. I got stuck once in the middle, but otherwise it was smooth sailing
The 1 was a naked single. it was available after you placed the 3 early on in row 2. That was the naked single I found when you found the 3 naked single which I had missed. The big trick was the 45 interaction that I missed completely. How did you spot that?
Ditto to this.
Very great solution, but I don't understand how you figure out that the locations of 4 on the column 1, and common 2. minute 13.00? thank you.
Finished in 16:21 ... but I can only claim a partial victory on this one. I spent 13 minutes getting as far as I could logically, then made a guess (at r3c9) which led to the end.
Tough classic!
EDIT ... I missed the 245-triple in c2, and then I missed the logic Mark used to place the 4s in rows 4 and 7.
I think this can be easily solved by considering the skyscraper pattern of the same 4s in row 3 and row 7!
25:36. Took about 15 minutes for me to get to the 'technique' part. Would of never found that unless I started to guess.
31:11 for my solve time. I broke through by eventually finding some empty rectangles.
The 3 in box 2 bugged me for 10 minutes xD..
I’m lost from 17:00 forward... :(
I feel like after placing the R1C2 pair of 2-4 (which I don’t understand how he got), it just gets more confusing. I’m a decent player, but I feel like I’m completely missing the logic behind the next couple of minutes of moves... ☹️
One thing that might be helpful is to, every so often, just go back to basic scanning, and methodically run through all the numbers. This gives, for example, the 3 in box 2 around 12:00.
Though true, in this case it doesn't actually bypass the need to do something advanced on 4s, such as the skyscraper that Mark found, or a two-string kite.
@@Rangsk Also true. I was considering editing my comment to include your point, but you got there first.
38:00 one of the first classics on this channel I do by myself
I don’t understand why the 45 squares in box 1 and 7 must be both the same. I mean both 4s or both 5s.
I did the 4 trick by using r3c9 and it can't be 4 because that would mean there is no position for a 4 in box 7 because it forces r1c2 in box 1 and r8c78 in box 9. Therefore r3c9 is a 9 and r3c1 is a 4 and basically the same rest of solve. Not sure what this method is called though.
I just saw your video. You made a logic error at 12:50 regarding the two 4,5 squares when you said they must both be four. In fact they can also both be five. The final solution will have a 4 in r3c9 and an 8 in r7c9. This puzzle has several possible solutions. I have created three others besides your solution.
There's no mistake at 12:50. You cannot have 5 in r2c7 and 8 in r7c9 since there would be nowhere for a 4 in row 7.
Can you share one of your alternative solutions?
Did you hit the check button to confirm it is a correct answer? (This being a standard sudoko grid and not a variant, the check button will work for validating the solution)
20:33 for me. Stupid X-wings. lol. And I'm terrible at spotting triples. Still, not bad.
Good Sudoku primer games
30 minutes here. Rather challenging; I got stuck for a while until I found an empty rectangle in 4 in box 1, after which it unraveled.
Can someone explain how he got the triple at 7:58? Seemed like a guess to me.
He deduced the possible digits in the cells in rows 1, 2, and 7 just by elimination from the same row, column and box. I'd do that too when I get stuck, but I'd just pick cells at more or less random and tediously work them through one by one. It'd probably take me half an hour to spot that. Brute work (or lots of pattern recognition training), no fun logic.
It did take me 28 minutes but I did figure it out myself. (Late because I noticed I am not, for some weird reason, really alert at midnight, and chose to do these the day after.)
Very nice l solve your Sudoku a other paper. Please send me more Sudoku by u tube
Thank you.
That's great. However I don't understand why you concluded that r3c1 & r7c2 had to be identical? (either both 4s or both 5s) Thanks for the solve.
Think about where the digit from r7c2 can go in box 1 ... Mark
@@CrackingTheCryptic Ohhh! That makes total sense now. I was so over thinking it. Thank you!
I needed the "4" hint
Great Puzzle!!
How do you spot things so fast? All I found for 5 minutes were useless x wings on 4s and 5s.
Its just practice. When you do a lot of Sudokus you will see the patterns at some point
13:08 You already missed another single 3 in r2c4 and this actually gave a proper X-wing on 5s in r2c2 r2c6 r7c2 and r7c6. But seeing this step with the 4s in r3c1 and r7c2 was the critical step to solve the puzzle. I missed that logic and just bifurcated when I got stuck.
How would I get in touch with Simon/Mark to submit a new unique Sudoku style puzzle for review and critique?
I think you would be best off joining the CtC fan discord server and posting your idea in there.
yeah maybe he’s doing all this advanced shit but ill still be proud of myself for getting that obvious 3 in r2c4 lol
[8:05] So, where does 4 go in rows 3 & 7? There can't be two 4's in c9, so r3c1 or r7c2 must = 4, which means that r1c2 ≠ 4, etc. (No need to first recognize that r3c1 = r7c2, of course.)
[9:00-9:11] I haven't watched Simon's video from yesterday, but somehow I already knew that writing the wrong digit in a sudoku isn't a good idea. Go figure.
[10:10] r3c9 ≠ 5.... .......................................... [11:13] 3 in box 2...
[12:05] Something about “missing information”. In a sudoku.
[12:59-13:03] Machines? Like infinite monkeys? Huh?
[14:17] “We've got nothing better than this, after all that?” The puzzle nearly solves itself from this point on.
[14:50] “6 goes in up there [r4c1], where I wasn't expecting it....” Huh?
[15:53] “It wasn't a naked single; it was a naked pair.”-which made the naked single a bit more obvious.
[16:02] “That's a 2-8 pair, which we can't do just yet....” "We" can't? Really?
Lol
Lew, you really went through some a' these
Funny lol :) 😅
[[Funny cool and nice, that is. Good ones, Lew South, lol. 😅
I found 3 different finned x-wings (didn't spot that 245 tripple tho)
Who was screaming, "Look at the 3 in box 2?"
Their was an x-wing with 4's in columns 2 and 9
Thats basically what he spotted. He just used a different logic
Aren't there three places 4 can go in column 9? (Rows 1, 3 and 7)
Gets cracking T 3:45
Is it called a naked single because all the other numbers are looking at it???
21:10 found it quite easy.
35:15
skip to 4:00
Totally lost me at the 245 triple.
27 minutes, solved by finding a chain.
The hard way.....
23:16
30:38 for me
that was a great sudoku puzzle
I've watched this 3 times and I still don't understand the logic of the 4/5 trick he did. Can anyone help?
(At 12:31 in the video)
Whatever goes in r7c2 (the lower highlighted cell) will be forced into r3c1 (the upper highlighted cell) in box 1, so they will be the same.
If they are both 5 then the only place for a 4 in both rows 3 and 7 would be column 9, where they would clash. (Looking along rows 3 and 7, every other cell is fully pencil-marked, and 4s would only be able to go in column 9 if neither of the two highlighted cells are 4s).
@@RichSmith77 Sorry but you are incorrect. You can indeed have 5s in both r3c1 and r7c2. This will result in a 4 in r3c9 and an 8 in r7c9. I pointed out this flaw in logic to the presenter. I had the same confusion as Adriana when I watched the video and felt the presenters logic was incorrect. I have actually created three different solutions with 5s instead of 4s. One must be careful to not jump to conclusions.
@@garyhiggins1641 At the position Mark has reached at 12:31 in the video, you would not be able to put 5 in r7c2 and an 8 in r7c9 because there would be nowhere left for Mark to put a 4 in row 7. If you think Mark has made a mistake before 12:31 in the video, can you please say what the mistake was?
I feel confident that Mark hasn't made a mistake before this point, because no-one in the comments has highlighted one. Also your assertion in another comment that there are multiple solutions is difficult to believe without proof of an alternative solution, since every puzzle they feature is tested, Mark appears to reach a unique solution via a logical path (no-one is pointing out a specific mistake in Mark's logic to reach that solution) and no-one else in the comments is claiming the puzzle has multiple solutions, despite multiple solvers. It seems highly unlikely that you alone would find the puzzle has multiple solutions, and be the only one to comment on it. Can you share one of these alternative solutions so we can check it?
I could never be as good as you are at solving these, but I was cringing a little bit at how the 3 at row 2 column 4 eluded you for a long time. As soon as you placed the 59, it was yelling out as a naked single. But again -- I could never claim to be as good as you are at solving.
32nd. 27:53. Frustrating for something so simple
23:10 Hard to spot but i could spot it after some 5 mins of stuck
I used a bunch of finned X-wings to get out of this one. EDIT: The 2457 quad in column 2 would have gotten me there a lot faster!
I don't understand how he got there for that quadruple, can you explain (at minute 7:28)? Why is the 4th field in 7th region, and not in 4th (where there's no 7)?
@@meeeroslav I don't understand your question; can you rephrase it?
@@rickwoods5274 oh sure, thanks for replying! :)
So his train of thought that starts at 7:28, about 2457 quadruple in 2nd row... I don't see why a 7 cannot be in 2nd row in the 4th box? Instead he solves it directly in the 1st box.
Either he made an oversight with that 7 (because he did mention a 2458 quad, and then corrected himself to 2457, so maybe he was still looking at that 8 in box 4), or I'm missing something about the limitations for a 7 in box 4, but I can't see what :/
@@meeeroslav I'm still not sure what you mean by "second row in the fourth box", but 7 can't be in any cells of the quadruple except the one in row 3. There is already a 7 in row 1, there is already a 7 in row 2, and there is already a 7 in box 7. Thus, out of the 2457 quad, the 7 must be the one in row 3.
If you're asking why a 7 can't go in row 6 (which is the closest thing I can see to "second row in the fourth box"), it's directly because of the 2457 quad; this is just how naked quadruples work. If you put a 7 anywhere in the column that isn't in the quad, then you have four cells that have to be selected from three digits, which breaks the puzzle.
If this doesn't answer your question, it would help me to understand what you mean by using rxcx notation (where rows stretch left to right and are numbered 1 to 9 starting at the top, and columns stretch top to bottom and are numbered 1 to 9 starting at the left).
@@rickwoods5274 column, I meant column 2, not row! 😭
So r6c2 is the one I'm wondering about, but you already guessed and answered it, thank you. I guess I need to read up on how quadruples work, I'm new to this, so I must be missing something.
Thanks again!
What's so special in this sudoku? I solved it in less than 3 minutes.
Completed in a very sedate 9m57s.
You brought me to a point, where classic Sudoku is really boring to solve because you don't have to think about a lot of constraints...
So seeing one of you solving a classic is like: "ahh they do this boring stuff for the newbies"
But then you actually realize, that you still lern new things every time :D:D
Newbie-ing myself indeed...
🤩
"Is there some sort of 5 swordfish?" No, but there is an obvious triple in box 2 which disambiguates the 5. Finds a way to bifurcate into the obvious
There was actually an X-wing on 5s, which eliminated at least one 5. The remaining 5s formed another X-wing and a swordfish (which included the 57 pair in box 5. I wouldn't have found them if I hadn't gotten the clue from the video. That didn't help much though, I was still stuck.
This wasn't bifurcation. It was a skyscraper. Just because he didn't know the name of what he was using doesn't mean it's bifurcation.
@@Rangsk I am sure he knows what a skyscraper is, I was being hyperbolic. I was laughing at him missing the obvious when he can casually glance at a row and instantly see the possible digits normally
First?
14 minutes for me. I didn't actually find it hard at all.