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Powered by LifeTypeThis 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.
I can't say how long it took me, because I was doing other things (like reading Janet's updates) at the same time. But I didn't find it particularly challenging. The first blank I filled in was the rightmost column, 5th row. After that the middle square of 9 came fairly quickly, and it was all slogging from there.
You won't like my method, however, because it starts with writing all the possibilities in each square. (The order in which I fill in the possibilities depends on the particular set up, so it's not all just slogging.) Once I've done that, certain things jump out at me, such as unique pairs. In this case, I could see that the first two rows of the rightmost columm could only be 5 or 9, so one had to be 5 and the other 9, thus when I found that the 5th row of that column could only be 3, 5, or 9, I knew it had to be 3.
I wonder if you find the 3 through 6 stars to be equally difficult because you don't do any "first pass" rules to quickly write down stuff. The level of stars is different depending on which book/paper the puzzle is in, so I don't really pay attention, other than not doing the "easy" puzzles like you said.
I have always done an explicit first pass without writing down anything except for numbers that I am sure of. It is a relatively quick process. Last weekend, I developed a new strategy, where I write down little numbers if I know that a "5" has to either be in this square or that square, within any group of 9, though more likely in a box than in a row or column. That rule is how I got the 5 and 9 in this particular puzzle.
I don't like pencils all that much for any kind of writing, so when doing a suduko, one sub-goal is to write down as little as possible, because I won't be able to erase it.
I know Porter works in an entirely different way, and so do you; I wonder how many different approaches there are? Then there's always Mark's.... :) I wonder if any are insoluble?
At Thanksgiving, I didn't feel like re-doing the whole puzzle to see if it would have worked the second way too.
It does have an interesting feature of checking whether there is more than one solution to a puzzle, so presumably that means that happens sometimes.