How to solve Ultra Fiendish sudoku - without guessing!

Simon shows how to solve this extremely difficult puzzle logically.

[@nlo114]

I get frustrated watching people entering into logical analysis before they have taken the free gifts on board, (like R3C2-8, R8C5-3). They may not be useful immediately, but speed things up later.

[@jdoe834]

Yesterday I did not know what an xwing was, then I watched a bunch of your videos, and today I found the swordfish and solved this puzzle myself in 14 minutes. You definitely leveled up my sudoku, and gave me a new appreciation for it. Thanks!

[@mander40101]

You missed 3 obvious numbers right off the start. I always go for the easy solves first and work from there.

[@sHillChannel]

I tried it before watching the video and was able to solve using standard sudoku procedure and 2/cell notation. I didn't use xwings or anything else special. Why did you not find the 8 in box 1 that was solvable from the start? That was the first cell i filled in.

[@kana2112]

So odd going back to the old puzzles and not seeing all the links that are in the descriptions of the newer ones.

[@jetranger57b]

There are a couple of obvious placements that got initially overlooked in addition to the initial 5 - the 8 in the top left squar. (must go in bottom center cell), 3 in the bottom center square (middle cell), 9 in the middle square (eventually got to that)

[@eireannsg]

Oh man, till the end you missed the most obvious one. The 8 in C2R3 Are you an 8 hater?

[@rodneygray4437]

HOW could he miss that 8?

[@erickguz]

It appears all of the experts have commented on the novice's approach...

[@BradAhrens]

The experts here reveal a lot about humanity.

[@davidson2004fatboy]

Y NOT ELIMINATE ALL THE EASY STUFF FIRST, THEN CARRY ON WITH WHAT YOUR DOING. I FOUND MANY NUMBERS U COULD ESTABLISH FIRST. YOUR MAKING THIS MORE DIFFICULT THAN IT NEEDS BE, RIGHT OUT OF THE GATE. VERY CONFUSED, I CAME HERE TO LEARN. STUDY PATTERNS AFTER ALL EASY NUMBERS HAVE BEEN SOLVED.

[@milkom2021]

I solved it with just a single Y-Wing in r5c7, r6c9 and r8c9

[@shehum]

The very first thing I saw was in the interaction of the given 8 in R1C9, with the given 8 in R2C6 coupled with another given 8 in R5C1 to give us an 8 in R3C2. Don't know why you spotted this only towards the end of your solve. You simply ignored all the 8s until towards the end! Spotting that 8 early on in the puzzle would definitely have been helpful, in my opinion.

[@familygarlick179]

Hi in box 9 how did you deduce that the number 5 was in colum 9 and not in colum 7, please explain as I can not see it meny thanks in antisipation Roger

[@chrismoyler]

Great solution, but sad to see you miss the 8 early on in row3 col2. Sometimes, focussing on the very tricky stuff means we miss the obvious!! Really enjoyed this solution.!! Thank you

[@eireannsg]

exactly, maybe he dislikes eights.

[@mikesummers-smith4091]

I was beginning to think that Simon must be allergic to 3s. Xwing - check; swordfish - check; two naked 3s - I'll get around to them later.

[@kellynlee6567]

There was an 8 too😂

[@SimonK91]

Before watching the video I tried it out myself, though I did not find the x-wing and swordfish, making it quite a challenge (Still not used to those two patterns). I am mainly doing it in two steps. First is to make a quick sweep of all the numbers, 1-9, all at once, repeated once or twice, then go through each 3x3 grid filling in all the possible values in each cell (this does not seem to be a good method, since it fills each cell with too many candidates). After that I try to eliminate candidates that does not fit. I managed to solve this sudoku by first finding a long XY-chain (not sure if that is the correct name for it) eliminating some candidates in bottom left 3x3 grid. Total time for me to solve this one was 47m 30s, though this video really helped me to change my method of solving sudoku-puzzles.

[@m2thef2thec]

Phew! Frustrating, after all the work you put in to sort out the sixes, and you didn't follow through on them. A bit of a haphazard solution, if I'm honest.

[@Anonymous-wb6yj]

You're missing the obvious

[@jbsoul4575]

I see why you wear glasses now. I'm the same way. That software you were using seems a bit antiquated compared to your new software.

[@dineshreddy3766]

hi... when u worked on # 6, why did u pick only those 3 columns and how? columns 1, 5 and 9.... when didnt you look for column 2 and 4?

[@CrackingTheCryptic]

I started in column 1 just because it's first! Now, once I have identified where a 6 can go in col 1, when I move on to the other columns, I must look for columns where the 6s can go in at least one common position with the positions they can go in col 1. So I did look at col2 (you can see the cursor moving as my eyes scan it if you look carefully!) but the open positions for 6s in column 2 do not overlap at all with the positions for column 1 so I move on. Obviously if by the time I reach column 9 I've found insufficient overlaps with column 1 then I'd need to discard col 1 as helpful and perhaps start with col 2 instead.

[@drferry]

I don’t know, but the way I can solve these is to put in all the obvious stuff, then fill in all the candidates in rows and columns that have at least five numbers in them, and see if that allows me to solve it. If not, I fill in all the candidates and go from there. Slow, but deadly.

[@Daedalus2010a]

Soon majorly frustrating. There’s a naked 8 in box 1... the 8s weren’t even looked at till about 19 min in. I don’t get jumping around between numbers. You’re bound to skip over important or naked singles. I start at 1 or 9 and work my way to that other end then back. Then go to working on patterns (x pattern, 3X pattern).

[@fvni]

No need to use any wings or fish. Find the 6-9 and 4-8 pairs in columns 7 & 9, isolate them, then the rest presents itself. No need for further notations. Can be done in 25 min. Good luck. Fun puzzle.

[@jimjimellell]

I solved it pretty much like Simon did, but I did not see the X-wing (as I am new at those). I don't even know how to spot a swordfish yet either. I DID see a connection between the 3's, 6's, and 9's, and, since the only 6's in row 1 were in col. 4 and col. 7, I tested the one in col. 7 because it also affected the 3 in row 1. By following Ariadne's Thread only a short way there was a contradiction that would break the puzzle. Using the correct 6 cleared up all the 3's, 6's, and 9's and birthed the 5-7 pair in col. 7 which just made the thing unwind - or, as Gandalf would say, "I smote it's ruin upon the mountainside". I am sure that there are a lot of you who have been doing Sudoku's for a long time and have noticed these repeating patterns and used them to help solve puzzles, and (like myself) never knew that other people actually gave names to these patterns . They are like the Theorems that I had to memorize in Algebra class and then forgot a week later after the exam. I've seen about 2 dozen different names for these "Techniques" and I am not about to memorize them all and then forget them a week later. I would not call this puzzle Ultra Fiendish, or even regular Fiendish. I think Simon was being cheeky when applauding the NYT for such a great puzzle and asking for more like that. Well Done!

[@craiggruenberg9591]

How did you arrive at the decision to "study this cell hard" at row 2, column 7 at 21:20 in the video? I have done thousands of sudoku and am quite experienced, but I am having trouble tracking your logic in deciding on that particular cell. And secondly, could another cell have yielded similar results in cracking the puzzle?

[@CrackingTheCryptic]

It's a while since I solved this puzzle but, looking at what I did, I agree with myself that I should have studied r2c7 hard :). The reason is simply that the x-wing on the 9s is almost useless in that it eliminates 9s from so few cells. In the video I suggest that only r2c7 is affected by the x-wing but I think r2c3 may also have had its 9 eliminated. That said, the fact that the 4/8 double is also appearing in col 7 certainly "feels" like it is the r2c7 square that's important. On your second question, perhaps, I'm not sure. I always try and extract every ounce of sweat from a deduction like the x-wing as I tend to assume (sometimes wrongly) that the puzzles have been designed carefully enough to have a "solution path". Today's video on the NYT Hard Sudoku (26 Sep, just posted) is similar in a way but less complicated. I found something early in the solve that I'm sure most solvers would have spotted but I'm stubborn enough to keep thinking about it to try and ensure I'm not missing how to make progress using the early spot...

[@craiggruenberg9591]

Cracking The Cryptic Thank you very much for your in depth description. It's always helpful to understand the "why" behind our feel or intuition for a particular decision. Thanks again.

[@rabidsamfan]

I got myself an entire book of blank sudoku grids just so I can try these before watching the whole video. Thanks! I'm really enjoying myself. Especially since our approaches are so different.

[@clickayna6157]

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[@jrbleau]

I do a "print screen" then select the grid in MS Paint and paste into MS Word and print it, solve it and compare my crude technique with theirs.

[@fritzhenning1]

I was screaming for the obvious 8 right from the outset, but thanks for unravelling the 6s

[@arunmunna8215]

Hi sir what is difference between diabolic suduko and fiendish suduko??

[@Keyboardje]

Took me quite some time to figure out the 69's and 57's but after that it almost solved itself. Took me exactly 43 minutes though. But... I did it never the less :D Nice learning experience with the sword fish thingy. Thanks!

[@joem6859]

Why didn't the 6's in columns 2 and 4 matter for the swordfish?? A little confusing...

[@nethe0]

Because they were in different rows to the 6's in the swordfish so they could still be 6's after the swordfish was completed.

[@pbegbie]

I don't know if I was being a bit slow, but I found the qualifying puzzles for the championships (completed with no intention of entering) trickier than the usual super-fiendish standard published by the Times. Perhaps that's normal? (I've no idea). But it would be nice to see you solve a couple of them logically now that the solutions have been published.

[@peterjongsma2779]

8!

[@paublusamericanus292]

I am still wondering about the 756 cell and knowing it was key. I thought sure the 9 xwing would go, and it just faded.

[@jeffroutledge9153]

I thought these puzzles were supposed to be played without any guesses/erasures?

[@FazalFariz]

That was easy though. There were shortcuts you didn't notice.

[@vijayavani5574]

Nice puzzle and enjoyed it.thanks

[@eddiebock2746]

I don't see why you put a 9 in row 1 col 3 and didn't put one in row 2 col 3. How did you eliminate the possibility of a 9 in row 2 col 3?

[@CrackingTheCryptic]

It is the highlighted x-wing pattern you can see at 7mins 45secs into the video. ie the 9s can only go in 2 positions in each of rows1 and 9. As these positions correspond, ie both are in columns 3 and 7 it is IMPOSSIBLE for there to be any more 9s in these two columns. You can try it for yourself but if, for example, there is a 9 in r1c7 then there MUST be a 9 in r9c3. And vice versa if the 9 was instead in r1r3. Therefore we can eliminate the possibility of a 9 in row 2 col 3.

[@tracymiller1149]

"We're starting to cook with gas". Ha, ha. That must be a British saying.

[@jimjimellell]

"Now we're cooking with gas" was a slogan launched by the American Gas Association in the 1900's and caught on around the world. Took was Peregrin"s (Pippin) last name . He lit the beacon at Minas Tirith with oils or pine tree extracts but no gas. Ha-Ha!

[@madFEAT]

Very Nice! I hardly ever see anyone use a swordfish to solve. :)

[@6stones]

What sudoku program do you use in this video? Thanks.

[@yadasampatidasa8690]

Why not first solve the obvious 8 (top left box) and 9 (middle box)?

[@CrackingTheCryptic]

These solves are done live. This means we don't always spot everything obvious! But it's "real" ie not pre-solved which means you see a real solving experience. Mark and I are happy to take on most people out there at speed solving if they think we're bad solvers :)

[@LinXiaoChaun]

Thanks for elucidating the "X-wing" and the "swordfish"!

[@truptil3087]

Thanks!! I'm exactly looking for this.. very well explained!! One query, how did you get 5 on 8th row, 9th column?

[@CrackingTheCryptic]

Delighted you enjoyed the video! The 5 can be placed at that point because, in col 9, we need to place [2,3,5] at that point and there is already a 2 and a 3 in the bottom right 3x3 block (in row 7,col 7 & row 9, col 8).

[@aku7598]

1st i see r3c2 is 8

[@rputran]

how did you get 3 on row 9?

[@CrackingTheCryptic]

It's because in the bottom right 3x3 block, there are two cells containing pencil-marked 6 AND pencil-marked 9. When we write pencil-marks on this channel, it means that there are exactly two positions that a number can go within a 3x3 block so, in this case, (row9, col7) and (row7, col 9) can ONLY contain either a 6 or a 9 (because these particular numbers can ONLY go in one of these two positions). Therefore, when I see that a 3 must go on the bottom row in the bottom-right 3x3 block, I know it can only be placed in row9, col 8. Does that make more sense?