Advanced Sudoku: X-Wing Into Skyscraper

We take a look at Diabolical-rated puzzle requested by one of our viewers (from The Daily Telegraph series). It is actually a beautiful Sudoku with a nice trick that can make it a quick solve. Early x-wing into a skyscraper pattern makes light work of it :)
#crypticcracking
#Sudoku
#skyscraper
#xwing

[@KayDubs77]

Thank you so much for your videos! I've loved Sudoku for years but never knew how to work the logic of the "harder" puzzles. I mostly just guessed and hoped for the best. But I absolutely love how you explain the logic and make deductions based on evidence and the rules.

[@antminehead]

Really enjoying your channel, thanks for all you do. I can just about understand the logic behind these more advanced solves but I'm nowhere near being able to SPOT them in the first place - that skill seems to be eluding me so far!

[@drakenskag]

What a satisfying puzzle. Thank you for the insight. I'm trying to wrap my head around some of the harder strategies, and your tutorials have been pretty helpful!

[@bidyutchakraborty2433]

What a keen observation you have demonstrated here. The effect of block 4 on the col 3 was a real gem of it. I wish I could do the same myself. I am a sudoku solver and solved many a diabolical problems I got from the Indian 'The Telegraph' news paper. But all those solutions took a lot of time and never I could mark the X-wing, Y-wing etc. successfully. Marvelous! I hope I can somewhat improve my skill now. Thank you for your guides.

[@donnyh3497]

Fantastic lesson on the skyscraper! I hope that I can find one in a puzzle now.😅

[@thornev7545]

I appreciate you showing the skyscraper. I read about it before and thought my head was going to explode. Now I have a pretty good understanding of how it works. Thank you!

[@sallywilliams3167]

You see things SO quickly. I really enjoy watching your videos and have learn't so much.......Thank you. I know you have been asked before but can you tell me which app you are using here.

[@TheGrifter247]

I only started watching CTC about 1 month ago - June 2021. Coming back to see the progress the channel has made is very interesting. Great job Simon & Mark!

[@vijayaragavand9474]

I enjoy watching your videos, learning sudoku solving technique. Thanks

[@goldenera7090]

thanks very much Simon. good solve. I am beginning to learn about skyscrapers :)

[@davidmaloney4860]

Excellent discussion on this. Very helpful.

[@MuhammadNabil-st6rd]

Thank you once again.

[@debralegorreta1375]

Powerful 5 at R6 C9 gave rise to both the x-wing and the skyscraper... IF you can spot it. LOL Well, you can spot the key 5 once you see that that single 5 so restricts the 5's to two cells in C7. What was really wonderful is your doubling down on that tactic on C5 where with the help of the x-wing again the 5's were restricted to two cells, With all this stress on the 5's it's not too surprising to then find that the 5's have been restricted to a single cell in R1 and imo that cracks the nut. Nice!

[@subhechchabanerjee3421]

Thank you for teaching the skyscraper technique 😍

[@jeffbrunton3291]

That 59 is a ‘trick’ I did not know, and now it’s so obvious. Cheers!

[@skyeako8703]

Thanx. Big help on skyscraper territory.

[@danohair7656]

great demo; nice application of Xwing and skyscraper interaction

[@stefanholbek2449]

I may have to stop commenting on every video ... but this is just BRILLIANT! And yes: You gotta yet ANOTHER subscriber! ;O)

[@martinpolach6171]

How wonderfully elegant

[@QWERTY-vv5xc]

Think I just cried for this. If I always go through process of elimination from 1 to 9 does that mean I will never get to experience this ?

[@dvoorganisation459]

Thank you Sir

[@vulcandj]

I tried this before watching you solve it. I was able to place the first three digits as you did, then I was stumped. Thanks for the explanation! I don’t fully understand it yet, but it should sink in. 😎

[@juliepokela506]

Very helpful, and the baby noises were adorable!

[@linazdll4101]

Think you. Sir Simon. I FOLLOW YOU FROM ALGERIA. TKINKS

[@rabidsamfan]

Nice. I am only just starting to get skyscrapers.

[@tglrod]

Genius!

[@worcesterexchange554]

I was struggling for a long time - and going round and round in circles wondering why it would not work, until I realized that one of the numbers in the sky-scraper could not exist. That was the key - After eliminating that number, the rest of it fell into place. Not sure why that happened? But it solved the puzzle.

[@hgb0005]

good god i just tried this and it took me forever......i was finally able to solve without guessing........i found the same important stuff......but it was like the very last thing i did/found..........jesus simon is so impressive.......i need to learn to scan and find things better

[@lawrencekallal6640]

With the 59 pair in C3 at 4:00, a simple way to solve this puzzle is to try putting the 5 in R7. Following the resultant forced 5's around the board there ends up being no open spot left for a 5 in the top left 3x3. So, the 5 has to go in R3. This is enough to crack open the puzzle.

[@QWERTY-vv5xc]

how do you eliminate r2c2

[@gdlikestoast1284]

^

[@hernandayolearyallda]

At 2:08, why couldn't the 7 have gone 1 block higher diagonally [beside the 5 and above the 9], why must it have gone next to the 9 in column 1? Like how do you know the 4/3 could have gone where you placed that 7?

[@vasilejosu3431]

Hello, what app are you using here?

[@stridedeck]

I think I saw a logical error. In your X-Wing with the 5, before doing the skyscraper, the two cells have 5-9, the other cells can not have 9 as then it will have two answers. At the end of the video, in the top corner 3x3 box, you placed a 9 where the original x-wing cell with the 5. If you look closely, you can exchange the 5s and 9s around for another solution.

[@danielspivak3926]

This doesn't violate uniqueness because the 5s and 9s are all in different 3x3 cells. Interchanging them gives a conflict with the other 5s and 9s in those 3x3 cells. You can interchange only if they're contained in a total of only two distinct 3x3 cells.

[@stridedeck]

@@danielspivak3926 So, uniqueness has restrictions.

[@danielspivak3926]

@@stridedeck It's just that particular setup of two pairs, in which you have the restriction that they have to be contained in just two 3x3 squares to avoid this case; The principle itself doesn't have restrictions.

[@stridedeck]

@@danielspivak3926 I understand. I was only hoping that even though it would creat different placement of other numbers, it still would lead to another answer with different combinations and placements of numbers. I understand within a 3 x 3, no other placement of numbers change. I tried to find another solution flipping the 5s and 9s around and it fails.

[@dudesz17]

How do you submit a puzzle?

[@AznUzer]

I’m having trouble spotting the reasoning for the 1-6 pair. Anyone able to explain?

[@bidyutchakraborty2433]

In the block 2, top row, there will be 1-6 pair. The reason being: 1) Due to the formation of skyscraper of 5's, cols 4 & 6 or row 1 cannot have 5 (either R1C8 or R2C5 will have 5) 2) 4,7,2 & 3 in row 1 and 2, 9, 8 in block 2 restricts these numbers to be in R1C4 and R1C6. So, only left numbers are 1-6 forming a pair.

[@hernandayolearyallda]

At 8:19, how does he conclude the 5 must go into that square? Did he miss see it or something and think it was a row higher than it was or is he just doing Sudoku so advanced I can't get it?

[@JennieFur47]

This was where I got lost as well. He doesn’t say the reason. Driving me nuts.

[@hernandayolearyallda]

@@JennieFur47 Yeah, he did an advance moved but didn't explain it well, I now understand it, after doing many advances sudokus for the last 8 months since I made the comment. Just seeing your comment and rewatching it. He is looking at the puzzle as blocks of what must go in a place or can't. You see Row 1 [the horizontal block across the top of the puzzle]. You see Column 1 [the first vertical column going from the left of the screen, it runs from the top to the bottom of the puzzle]. In Column 1, Row 4, there is a 5. That 5 prevents a 5 from going into Row1, Column 1 [R1C1 for short]. As remember, in Sudoku you can't have the same number 2x in the same row, column or box. In Row 1, Column 4 and 6, he narrowed it down to a 1/6 combo pair. Ie. 1/6 | 2| 1/6. So 5 cannot be in box 2. That only leaves box 3, or Row Column 8 or 9. Because there is a 5 in row 6, column 9 [or the bottom of the 6th box on the bottom right side of that box], you can't have any 5s in the 9th column. Therefore, the only possible spot for a 5 in row 1, is column 8, where he put it. And as a reminder, the 1/6 came about as a result of the skyscraper which we said showed the 5s had to occur in a combo of those highlighted spots. Which just means the 5 has to either be the green 5 in box 2 or the purple 5 in box 3. And all the other numbers that aren't 1/6 were either in that box 2 or row 1 already.

[@GerritHaase]

Can this be done in the webapp?

[@qilwisy3429]

Inknow this is an old video but i do not understand the logic behind the 7 at 10:00

[@LifeisReal1990]

The vertical 3 cells in column 2 are candidates 2, 5, or 7. In row 7, you can see a 2 and 5 already, making the only choice 7. The 1 and 3 pairs means the bottom 3 cells cant be 1 or 3, cuz then the top cells wouldn't finish. Same reasoning behind the 6 and 8. So up above, you see 1, 3, 4, 6, 8, 9, leaving 2, 5, 7.

[@MrAlivallo]

I have no idea what he's looking for. Xwing + skyscraper what relationship? Need more arrows and logic definition

[@moshehim1000]

Hello, I have a couple of questions: 10:40 - why does cell r8c5 has to be a 5? 5 could alternatively go at this time in cell r8c4 and r9c4. I know these cells aren't affected by the x-wing. I'm thinking maybe the skyscraper has something to do with it? If so, I don't know how. 3:28 - In the x-wing, there are 5s in all 4 cells. and 9s in two of them. How come the uniqueness problem doesn't preclude 9s from being also in the other two cells? (I know it doesn't, because other than you not mentioning it, in the end one of those extra cells actually does hold a 9.) I don't understand when it applies and when it doesn't. Actually, I don't quite understand the logic behind it at all, only that it works... 🙂 Thanks.

[@hgb0005]

first question: yes it has to do with the skyscraper......after finding the x-wing.......now column 5 only has two spots for a 5.......he placed a 3 in upper spot (row 2) leaving the only spot for a 5 in that column where he placed it (row 8, thus eliminating the possibilities in column 4)

[@hgb0005]

second question: (now I wouldn't rely too much on me for this one......i only recently encountered uniqueness and that was from the comments not an actual video) I think you are incorrect in saying you know it doesnt (preclude.....) and i think the key word is 'both'. I believe that uniqueness DOES preclude 9s from being also in BOTH the other two cells, but it doesn't preclude 1 nine from being in 1 of the other 2 cells. if all four of the cells of 5 xwing could only be 5 or 9s, then i believe the puzzle would have what Simon calls a deadly pattern (you could have 5s & 9s on the diagonals of the x-wing or you could switch the positions of those 5s & 9s and it wouldn't affect the rest of the puzzle. Therefore there would not be a unique solution.

[@moshehim1000]

@@hgb0005 Ah. Thanks.

[@moshehim1000]

@@hgb0005 Hi, Of course 9 can't be in both cells - they're in the same column, that's basic sudoku rules. But in other cases, in other videos, where you get an x-wing situation like this one, Simon eliminates the number in both cells because of uniqueness, and while I never quite understood why, it does seem to work and to lead him to complete solutions. Now, in saying that that one 9 can be in one of the two cells, you have to mark both cells with pencil-marks for potential nines. Which is exactly the situation with uniqueness that has lead Simon to not placing those marks down in other puzzles.

[@hgb0005]

@@moshehim1000 yes...i didn't word that well.....(also again Im certainly not an authority on this)...i guess i meant uniqueness precludes all four of the 5-x-wing cells from being only 5s or 9s as possibilities.......because then that is the deadly pattern....and so on as stated previously......now that may not even make sense in terms of solving the puzzle at that point in the vid.......but i guess that was meant more as something to consider/think about in understanding uniqueness

[@mjanvn2062]

How do you decide that the 5's for the X wing go into colum 3 and 7? 3 and 8 would have made the same pattern ... I've been looking through so many tutorials now and always end up with the same question 🙈 driving me crazzzyyyy

[@haraldpuester6205]

For a X wing you need 2 columns (or rows) where only 2 positions are available for a number, and this 2 positions must be in the same row (column). In this case a 5 in column 8 would have more than 2 possible positions.

[@simmo5071]

@@haraldpuester6205 The logic of how an x wing is not the question. Rather how did he eliminate row 8 from having the 5 also?

[@wossaaaat]

Found the x-wing quite quickly, but couldn't see how to convert it. Stared for aaaaaages without making any progress beyond more pencil marks. Came back and just saw the title again, and was like... _wait... naaaaah.... can't be.... but.... but..... NOOOOOOOOOO!!_ Can't believe I missed it for so long. Piece of urine once you get that bit. Thanks once again for showing me where I'm being blind.

[@dvoorganisation459]

Thank you Sir