This is the hardest one I have ever worked on. [Update 10/24/2006: thanks to you all reading this post and posting other links, I have since found more interesting ones -- look through the comments - the one posted on October 9th appears to be pretty hard, though maybe I just made a mistake on it]   When we were with my parents last weekend, we found out that my parents are big Suduko fans, and do quite a number of them during the week, in different papers, page-a-day calendars, etc.

Dad was trying to find a hard one for me, and found a "six star" one, that turned out to be not that hard - reasonably difficult, but similar to what I had seen before.  We had been trading secrets of how we figure out puzzles the fastest, and he liked one of my starting first-pass rules, but then found a puzzle that using his rule, and then my rule still did not find a single number during the first pass.  After that, the only step I know is to start writing down all possibilities, and it is sort of interesting to see how fast you can narrow them down, it is a little too brute force for my taste, and so I am not interested in it as much.

Once you get one number on this puzzle, it is a normal, reasonably difficult sudoku, the trick is getting the first number.  So, I would be interested in hearing how long it takes you to get the first number filled in, and if it doesn't take you that long, what is your strategy, because it must be different than mine.

(see my comment on 11/27/2006 for the original puzzle)

Edit: Don't read the comments if you are interested in solving the puzzle, wait until after you are done, or at least until after you have found the first square -- which Linda and I probably had the same first square, so that leads me to believe that everyone might have to start at the same point.

Posted by Jon Daley on April 28, 2006, 9:21 pm | Read 68865 times
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Thanks - I'll try it out. I have not seen any that needed true guesses and I have doubted whether they exist.
Posted by Jon Daley on October 9, 2006, 3:03 pm

I downloaded a sudoku solver program (from Gwerdy software) to see if it worked. After a few tests, I wanted to look for a very hard one. This was the first sudoku I found by searching. This program (on my computer, which is not really fast) took 1,089 seconds to solve it...
Posted by Erwipro on October 21, 2006, 9:16 am

I have talked with a couple people about the possibility of requiring a guess, and we all think that guesses are not needed by definition, because a sudoku is supposed to only have one solution.

The trick is that you have to know harder and harder rules, so at some point our brains have an easier time just guessing, rather than learning really hard rules.

I have been working on your (YH) first puzzle, and it is pretty difficult. My father has gotten along farther than I have, though I haven't been able to figure out how he made one leap in logic, I think he might have missed something, but it happened to be okay.
Posted by jondaley on October 21, 2006, 9:39 am

i was happy when i typed in "hardest suduko game" in yahoo search and found this one...but was greatly disappointed when i actually did it...the atlanta constitution offers more of a challenge with its 5 and 6 star puzzles....i might even go as far as to say the 3 and 4 stars are harder than this you said once you find that one it all comes together...the problem i had with it was i found it way to fast...perhaps finding it so fast was just luck but it made the entire puzzle way to easy for my liking.....thanks for the post though...please post more that give you trouble, because next to the monday through friday puzzles in the ajc it was fun doing it
Posted by Kap on October 24, 2006, 3:03 pm

I added an "update" to the original post to say that you have to read through the comments to find the harder ones, I realize that the original one I posted is not as hard as I originally thought.

Though I don't think I have seen a puzzle that I took longer to find the first number.
Posted by Jon Daley on October 24, 2006, 6:40 pm

i got it in 3 mins 22secs
Posted by Samuel on November 6, 2006, 2:38 pm

I assume you are talking about a computer solver? If not, I am pretty impressed...
Posted by Jon Daley on November 6, 2006, 4:45 pm

Oh, or maybe you mean finding just the first number. Yeah - I can't remember how long it took me, longer than that. I might stand corrected - I forget how long it took to me to get the first number on the puzzle posted by YH on October 9th, on this page.
Posted by Jon Daley on November 6, 2006, 4:47 pm

Hi - I wrote a solver in Excel (VBA) available at It solves the puzzles listed here in a second or two; they are in the built-in library as #260, 261 and 262. I'm new to all this and wonder if anyone has a source of more difficult puzzles. Most are solved with logic only. If that's not enough (e.g. try #81 in the solver), it doesn't guess per se, but it does choose sells to "look forward" to see if only one candidate doesn't lead to a conflict elsewhere, or if all candidates lead to the same result in any other cell. I believe these tests are equivalent to more advanced strategies such as chains, coloring, etc.
Posted by Andy on November 8, 2006, 11:53 pm

Never was a sudoku fan myself, I understand the logic but I find the process itself a frustrating, labourious and time-consuming task. I do however enjoy writing programs, which provide the satifaction of logical thinking without any tiresome human inadequacies to get in the way. The program I wrote for this was in basic, and follows the following fool-proof method for solving any sudoku puzzles: 1. Employs basic logic, to deduce as many squares as possible - similar to a human filling in the possibilities and deducing what must be true. 2. When first step has been exhausted; eg after an entire repetition no new values have been obtained program selects a square with as few possibilities as possible (bit of a mouthful) and makes a 1st generation guess which it records in an array. 3. Returns to step 1. 4. Checks for consistency, eg if any rules have been broken. If ok then 2nd generation guess is made if necessary, if incorrect then erases all current generation guesses and makes new guess. This process can repeat indefinitely moving up and down generations as appropriate if all possibilities in a certain generation have been exhausted. 5. Blah blah blah you get the idea within a couple of seconds either you obtain a solution to the puzzle or you're informed that the puzzle itself is at fault and there are no solutions. In this particular case my program only had to resort to a 1st generation guess so by computer standards this puzzle is pretty tame.
Posted by Scott Fleming on November 23, 2006, 7:10 pm

There are palm programs for sudoku. The one I have allowed me to solve this puzzle in 3.5 minutes. It allows the use of pencil marks. The program itself solved the puzzle in 10 seconds.
Posted by niknar mot on November 27, 2006, 9:18 pm

I am moving the original puzzle to a comment, since I think everyone keeps commenting on this easier one, rather than the hard ones talked about in the comments.
14 3 8
8 6
7 96
1 8 7 2
24 7
9 4
8 7 92
Posted by jondaley on November 27, 2006, 9:44 pm

I agree that this original puzzle is NOT hard. I have just written a Sudoku solver in ANSI C based on a very simple algorithm. It finds the solution of the original puzzle without ANY guess. The 2 puzzles presented by YH are also found to have unique solutions, but require some guessing (7 and 4 guesses, respectively). If you're interested in the source code, send me an email at humbertoldi at yahoo dot fr.
Posted by Steffen on November 28, 2006, 11:25 am

Can anyone answer the question about what are guesses? Does that imply non-unique solutions? Why would you ever need to guess? What happens if you go back and guess a different value than you guessed the first time?
Posted by Jon Daley on November 28, 2006, 11:36 am

That depends on how well the puzzle has been constructed. Proper puzzles should only have one solution but it is of course possible to construct a puzzle with multiple solutions though in general it's an undesirable characteristic. Guesses are simply equivalent to higher forms of logical deduction and the use of a guess does not necessitate multiple solutions.
Posted by Scott Fleming on December 3, 2006, 11:09 am
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