The Simple Trick For Hard Sudokus

[Music] hello and welcome to sunday's edition of cracking the cryptic and on screen today we have a puzzle suggested to us by one of our viewers uh this came in from kevin morford who describes this as his favorite ever classic sudoku which is praise indeed given that we've all probably solved quite a lot of classic sudoku's in our time and this is from a book he owns called extreme sudoku now we don't do very many classics on the channel and one of the reasons is that it's um it's quite difficult to find really elegant classic puzzles um they tend to if as they get more difficult they tend to uh verge on sort of needing computer assistance to be solvable and what we and we don't like that we don't want stuff that requires bowman's bingo bifurcation or techniques not spotted by human beings what we want is things that can be subjected to cunning now in terms of whether this puzzle is subjectable to comic to to cunning i'll untangle my teeth in a minute um mark has tried this and found it very difficult um but uh he knows as well as i do that we're not gonna show stuff that needs bifurcation so i'm guessing he's passed this on to me because um yeah well well it could be one of mark's little jokes but it's more likely to be the case that there is just some trick that we need to spot so hopefully we'll be able to do that and one of the things i think that's interesting about this puzzle in terms of it being challenging is that there are a lot of given digits in this puzzle we've gotten nine and two lots of eight so there are 20 uh 25 digits in this puzzle so it's way over the minimum for a classic which is 17 and tends to suggest it might not be too hard but anyway we shall see you must have a go as well obviously now before we get started couple of things to mention firstly our competition to find the best solution to sam kaplan lines is brutal puzzle is now over thank you all for submitting entries and well done to the hundreds of you who solved this puzzle correctly absolutely brilliant um we've sent a short list over to sam to ask him to judge which he thinks is the sort of most faithful to the logic that he was thinking of and also makes the best artistic impression on him and he's going to get back to us in the next day or so in the meantime if you did struggle and you want to know the logical path then you can watch the man himself you can watch sam um actually solve his own puzzle on patreon at the moment we've got a patreon bonus video we've released it today i've watched it it's a lot of fun and yeah it reveals i'm revealed two things to me firstly how clever sam is for setting such a clever puzzle but also when you see extremely intelligent people talk about difficult things the fluency with which they do it is so impressive so yeah i i enjoyed watching sam talk through uh talk through that sudoku now these are also halcyon days if you like our crossword content because mark has been busy as you know practicing hard for the upcoming times crossword championship which i think is next weekend um we're they're doing an online crossword competition this year and mark's been putting his practice sessions up on patreon and there's another one that went up yesterday so so he's now done two of these videos i think the first one i preferred because he struggled mightily on that and that was very very amusing to watch but if you do enjoy crosswords then you can check those videos out and we have just hours left now it's counting down on hours on our kickstarter campaign to create the greatest sudoku book in history um it's it's going to be a bit sad for us when it comes to a finish but um kickstarter backers have already unlocked the 27 um videos that mark and i are going to make where we blind solve the new puzzles in the book so that should be quite interesting and i have a feeling that they may even manage to unlock um the pdf version which is what we're offering at the next stretch goal so it's going to be an amazing project for us it really is and um please do support it if you feel able to i'll put a link on the screen here now in terms of going through the rules of this puzzle today it's very easy we just say normal sudoku rule supply there i've done it um so the way to play the puzzle if you want to have a go is to click the link under the video with that i get to play let's get cracking and see how we go um so there are only two ones in the grid that's not going to be the place to start probably um on classic sudoku i will only pencil mark in boxes if a digit can go in exactly two positions i don't allow myself the luxury of three positions or more when i'm doing classics our twos are a little bit uh see this worries me this worries me when these pencil marks are all offset like this and they're not in the same rows and columns it suggests there might be difficulties afoot [Music] let's i do hope i'm not being trolled again let's keep going threes are very on threes threes on the diagonal here but none of them do anything well none of them do much that's useful anyway um fours again this reminds me also of derek neal's slot machine technique have you ever looked at that because all of these digits seem to be offset by which i mean that they don't overlap in a set of three columns that are together like this um the you know these fours are all off place and they don't overlap in three rows either [Music] and it doesn't make it easy to spot pencil markings there aren't many fives in the grid okay this is going to be tricky uh sixes how many sixes oh we have got three sixes again all offset in the grid look sixes have got to go into one of those two cells sevens seven's all down this diagonal again all offset sevens can be in one of those two positions wow and again one of these two positions just you know oh eights are there are three eights of course they're all offset again kevin you are a naughty person sending this in this is when people describe classic sudokus as their very favorite ever it does make you wonder about them what does that uh nines actually a line here for once we can we can at least place nines in the same row in box nine is that all we get that's all we get from just scanning the digits we get no digits at all we get no pencil marks apart from these useless nines in the same row or column um so what can we see here can we see anything so let me explain what i'm doing at the moment what i'm doing is i'm scanning the digits as i go through so for example i've noticed this two and this two and i've asked the question where can two and this two where can two go in row column one of the grid so i'm seeing that two can only go in two positions um now when i see this my next step is i scan across and i try to spot where there's a similar restriction happening in any other columns of the grid i can i lock the twos into a similar into similar rows and unfortunately i can't so that's not proved helpful but that that's the sort of thing i'm looking for at the moment right now the other thing i'm noticing is the threes and the sevens are aligning [Music] um yeah okay okay so if we ask actually that is a useful point so there's a hidden pair on threes and sevens in i think it's this row by the looks of things so if you look at where seven and three can go this three seven rule out those two squares this three seven rule out that square and this three seven rule out that square so we know threes and sevens can only go into those two positions and this is called a hidden pair because you could not identify this by just focusing on the cells themselves if we look at this cell and ask what it can be and don't take into account the contents of the whole row but just focus on the cell you could see you could eliminate one two four five six and nine so this cell without spotting it's a hidden pair could still have been an eight but now it can't be now yeah there's something going on here with with twos and sevens as well this central box i think is really interesting because look if you try and if you look down the rows and just keep a mental track as you're going is there any pattern you're seeing repeated now you can see sevens and fours repeating there so that's one thing to keep track of fives and threes repeating twos and sevens repeating but the twos and sevens are really interesting because of the central box and not much else so yeah so when you see this this pattern here this is obviously relevant to rows it's eliminating twos and sevens from rows and this two seven here is eliminating twos and sevens from these rows as well so where can two and seven go in column five of the grid so there's definitely a theme to this puzzle it's the communication between the seven threes and the twos around the grid um [Music] now what we've actually got to be able to do here is somehow use this hopefully to make progress and again this is a hidden pair again so one thing uh one thing we should we should do when we've got this obviously is we should scan down the column and check whether it's eliminating we should do the same thing in row six so let's do that we've got one four five and nine to place so this square can only be five or nine um i mean i could i i will come back and pencil mark options if they can only be one of three digits if we need to but let's leave it at two digits for the moment david mcneil one of the uk's well the uk's best sudoku solver for years and years always accuses me of putting too many pencil marks in the grid when i start pencil marking three things in boxes so we shall please david by being um efficient with our pendulum pencil pencil marks one four five and eight along here so this one can only be four or five that one's c's four and five so that one can only be one and eight see this isn't this doesn't seem to be the trick that we needed to get started oh that's a shame because that was very beautiful ah one thing which the other thing we should do actually if you find a hidden pair it is important to obviously imagine or look at the effect of the hidden pair on the row for example this 37 is eliminating three seven from all the open cells but also it can be important to check it in the columns as well because this square used to be able to be an eight before we discovered this pair so uh now the fact it can't be an eight i'll no it doesn't affect anything but but this is the sort of thing where the elimination of an eight from this cell could have been relevant let's check this one so before we found this pair what could this have been it could have it couldn't have been one two it could have been three obviously it couldn't have been four it could have been five i think so five five has been restricted it couldn't have been six seven we know about uh right so so for this square it's the 5 that's been eliminated from it now is that helpful so i'm looking at the column now i'm saying the fact that the 5 has been removed from here is it helpful and the answer is ah no it doesn't even get it down to a choice of two it's the same as the eights up here so five is not useful um let's try the same with the the two seven we've looked at the two seven in the column let's look at it in the row so before this got eliminated down to just being two or seven it could have been it could have been one it couldn't have been three four or five all six seven we know about one or nine for this square so this not being able to be one or nine is that helpful no i don't think it is there's just not enough going on with ones in the grid or nines at least not up here oh dear this is not going very well now let's check this one we've got um so before we got the two seven pair this could have been one four five let's see that the two seven here at least it's eliminated more than one digit from the cell one four and five were all previously possible here six eight and nine were not so one four and five got eliminated by finding the two seven pair so one four five can we do anything with that ones are not propitious are they so one ah four oh four four is four is perfect four four these are these fours looking at those squares so eliminating four from this square did cause uh a restriction if we look at row um row eight of the grid before we found the hidden pair we didn't we would have known that four had to be in one of two cells but now it can only be in this one so this is a useful thing um let's just check fives as well sorry while i'm here fives no okay fives are no good fours therefore have to be in one of those two we've got a digit in the grid well done me um [Music] no no this was this was one of the squares the twos could go into in column one we looked at this before two could go in here and here so now the four has taken one of the two positions of twos so this has to be this has to be a two i think there's nowhere else for a two in that column i'm putting it in two this is going to go yeah this is going to be useful too as well because not only can i pencil twos down there now but look this two is seeing that pencil mark two so we get a two here we get a two here by sudoku just these two is interacting this two is seeing this so that's gonna um well this low this it's now it's now suddenly getting easier this has to be a two at the top by sudoku therefore this has to be a two this has to be a seven that's not two anymore i think all the twos suddenly get done in the grid and the sevens now look we can pencil sevens down here but these sevens are pointing one one of those two squares where there is a seven in the column and that seven takes the position of a four so suddenly we are cooking with gas here that seven gives me this which is means that must be a three over there that must be a seven that must be a seven that all the sevens have just got done fours have got to be in one of those so i've got to keep track of fours and threes now oh forza falls look like they're running out of gas don't they um [Music] so i can see that that's a five nine pair by scanning down column one so this is a six eight pair which means this square should be we should be able to get that yeah that's got to be a three and that three there is pointing that that's a three so threes must be here three here three oh yes three here and wow okay so suddenly we are doing remarkably well where does eight go in this box it's got to go there that means that's an a that means that's an a this puzzle's just suddenly all of these offsetting pairs have suddenly become useful um now that means we've got to put nine and one in here one's got to go in one of those cells nine has to go here now um so these two squares might be gettable might they they've got to be five and six yep five and six can go in that must be a four that plonks four at the top that squares a five or a nine i'm oh no it's not a five that's a nine that's a five that's a nine these two squares have got to be one and eight which we can do the eight goes in eight and six this is six i'm now wondering i'm i'm thinking i'm wondering whether mark didn't find those pairs because if you don't find those pairs based off the fact that these all everything that we were getting was so weirdly offset i can well imagine this puzzle would be an absolute nightmare and he will he will just have bifurcated it i know him that's exactly what he'll have done um and for that i give him no credit um now let's have a look down here what do we need come on simon where's your scanning uh it's going to be a four so these two squares are one and five which surprisingly i can't do but this square should be gettable that's just six i think six means that's a six means that's a nine if i trust my pencil marks i'm always loathe to do that if i put them in a long time ago for some reason this should be not an eight it's going to be a five that's got to be a one that fixes the one and the five the one goes in here that should be a five check and that is how to solve that puzzle now what a fascinating puzzle now kevin i'm wondering why this is your favorite sudoku i think it's because it is very elegant actually this 237 in the it's almost a puzzle you'd want to try again without spotting this just to see whether it is an absolute beast i'm going to be fascinated to see the comments actually so i'm no doubt there'll be some people who didn't notice the effect of this 237 and the fact you've got these three sevens and two sevens sort of pinching points in the grid if you don't spot that how much of a monster is this let me know i hope you enjoyed the solve and come back later on for another edition of cracking the cryptic

Channel: Cracking The Cryptic

Views: 424,118

Rating: 4.7960734 out of 5

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Id: IFHOtse62Jo

Length: 21min 55sec (1315 seconds)

Published: Sun Nov 15 2020