Tanks for the pencil tip, I’m not a huge fan of sudoku but I do it sometimes and it might help make my life easier.

I’m sure I had some other trick to bust out when the going got really tough… can’t remember what it was though.

german c’t magazine had a very nice article in issue 20/2006, and the author discussed the mathematics required and the backtracking algorithm he used quite nicely. The article came with a nice sudoku software for playing and solving, written in C#, and it requires .net 2.0+.

@Dax – yeah, these things do come up. They never did for me though.

Also, I didn’t know about Kakuro. It’s like a crossword puzzle… Only without words. Heh!

@Matt` – Ah, good suggestion. I didn’t think of that. Thanks – this actually improves the human based algorithm a bit. :)

I actually ran into a “graph problem” in my work once. I boiled the problem I was trying to solve down to an issue of topological sorting. Funny thing was that I didn’t need to implement a graph to accomplish it.

As far as Sudoku goes, I’m not much of a fan. I prefer to solve Kakuro puzzles.

I used to use the “pencil in tiny little numbers then work by elimination” method. One thing that might need to be considered that you didn’t cover up top is the effect of pairs/triples.

If 2 boxes in one column have the same 2 numbers left (and only those 2), then that pair of numbers must be in that pair of boxes, and so those numbers can be eliminated from the rest of the line.

You can also remove all other numbers from a box if its one of a pair and there’s nowhere else for the 2 numbers to go, which may mean one other box is left as the only possibility for one of the numbers you removed.

Some of the harder ones will require you to make a choice somewhere and follow it through, while being prepared to go back to that point if it turns out that was wrong, some will have multiple dead ends and possible trails to follow.. I don’t like those ones, feels too fuzzy and not logic-y enough. The Times’ puzzles are all pure logic though