Hardest Sudoku Puzzle?

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 91392 times
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Not a hard one, at least not as hard as I expect. It look clueless at the first glance, but my practise is to fill in all the possible and then it will be easy for you to sort out the possible start. Just like this puzzle, once you do so, it's pretty clear that where the first three numbers can be put down, and viola puzzle solved within 10 minutes.
Posted by Samson on May 10, 2006, 12:57 pm
When you say "fill in all the possible", what do you mean?
Posted by Jon Daley on May 11, 2006, 9:06 am
i havent done it yet, but no really hard ones i also fill in all the possible answers. Getting just one number is really easy in this one. I looked at it for a second and found one. I dont know if it gets harder but finding one is really easy.
Posted by sagar on May 11, 2006, 2:58 pm
You looked at it for a second after you filled in all the numbers, or just looked at it without writing anything down?

I am not interested in solutions that involve writing every single possibility down -- apparently lots of people solve Sudoku puzzles that way - a coworker said he isn't interested in Sudoku puzzles since you can just do that, he doesn't care about making improvements to his algorithm at all.
Posted by Jon Daley on May 11, 2006, 3:01 pm
You may not like doing that but sometimes it is a must. Go to this site and try doing any of them without writing anything down. It is hard even with writing all the posibilities down. http://www.sudokusolver.co.uk/grids_veryhard.html I think the 1520 difficulty one is new but i beat the second one with a difficulty of 976. Try them and see if you can get it. Its harder then you think. I think that most puzzles in newspapers are around 100-150 for hard puzzles.
Posted by sagar on May 11, 2006, 3:10 pm
sorry i forgot to answer your question. I looked at it for a second after i filled in all the possible values.
Posted by sagar on May 11, 2006, 3:11 pm
i just checked the difficulty of this puzzle and it is a 126.
Posted by sagar on May 11, 2006, 3:13 pm
Yes, I agree that sometimes you have to write down all the values - it is more interesting to me to find algorithms that simplify this process, instead of just brute-forcing it.

So, I guess the title of this post might be misleading - but if you write down all possibilities, all puzzles are solvable easily enough, right? I will take a look at your example and see what I think of it.
Posted by Jon Daley on May 11, 2006, 3:17 pm
Well, I still say it is more fun to not just write down all the possibilities first, but instead start with doing different algorithms, the 1520 or whatever the score is on the page you (sagar) mention, is pretty tough.

I had other work to do, so I stopped after working on it for a couple hours. I knew a bunch of spaces had to be one or the other, but only had written down one for-sure number.

I submitted it to a sudoku solver and watched it go through each step - the key step was a step it called "swordfish" where it looked at 9 squares at the same time, and then was able to cross out a value in a tenth square.

I was happy to see it go through most of its rules and not find an answer.
Posted by jondaley on May 11, 2006, 10:06 pm
Where is the 1520 puzzle? When I go to the link the highest level I see is 976.
Posted by SursumCorda on May 12, 2006, 2:45 pm
Hrm. I don't know how that site works. The puzzle is still in the #1 place, but, yes, the score is now lower.
Posted by Jon Daley on May 12, 2006, 3:05 pm
I can't tell what this is, might be interesting.
Posted by Jon Daley on May 15, 2006, 4:15 pm
Well, I solved it -- the 1520 or 976 or whatever. It wasn't that hard to get one number. The puzzle bothered me a little because, unlike other Sudokus I've done, it was not symmetrical. Oddly enough, the first number I found took care of that problem and made it symmetrical. Not that that made it any easier to solve. I didn't ever get a second "for sure" number until I got all the rest. That is, I narrowed it down as far as all my algorithms could do, and then started the "pick a number and follow it until you get a contradiction" rule. In the process I found a few mistakes I had made, but correcting them still did not allow my algorithms to find a certain second number. Nonetheless, after I corrected the mistakes, the first experimental path I chose to follow did not lead to a contradiction but rather to the solution.

Now to get back to real life....
Posted by SursumCorda on May 16, 2006, 3:30 pm
I am not sure if I follow - you had a choice of two numbers, and you just happened to pick correctly?
Posted by Jon Daley on May 16, 2006, 3:39 pm
Sudoku puzzles are designed so you dont have to guess and check. You can solve the 976 difficulty puzzles with logical reasoning. It is hard and sometimes frustrating, but it is fun in the end. That is if you figure it out.
Posted by sagar on May 16, 2006, 4:59 pm
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