I suspect most of you have seen these “logical” puzzles in many newspapers (not to mention little books). They consist of putting numbers into squares so each big square of nine squares has every number, one through nine. Likewise, so does every row and every column.

The easy ones are too easy, and the hard ones are substantially impossible. But the moderate ones are fairly challenging and satisfying. Sudoku is a big time-waster, and I won’t recommend anybody to take it up. But if you have some time to burn, well, sudoku will take up a lot of it. Each puzzle may take 20 to 60 minutes—if you don’t make any mtakes.

How come my wife can do a puzzle, but when I try it and get stalled, she can’t show me where to find the next legal move? A move that she surely just made? Maybe she can’t recall where to look for the next move. Last night, I was working on a fivestar puzzle, and I found it easy. She started on the same puzzle, and she got stuck! She could not find the next legal move.

It took me about nine minutes to find the legal moves that would let her continue. It was, of course, in the ~ last place I looked. She had overlooked a couple of small moves. Then she had no problem finishing it. But it sure was challenging for me to find that move!

TIPS AND TRICKS

I hate to guess. Many very difficult or
five-star puzzles force you to guess,
because after a while, there are no unique legal moves you can
make. (Or, to be precise, too many legal moves, and none that is
uniquely permitted.) I have looked for advanced sudoku techniques,
but most of them aren’t helpful.

Recently I’ve decided to cheat a little and put in one number from the published solution. I can usually finish most of them if I make a reasonable guess of which number to choose.

Some books recommend using “Ariadne’s Thread,” which is just a form of guessing. You make your guess and see if the puzzle plays out. If it doesn’t work, you “back up” to where you got stuck and started guessing. I do this by putting a small number in the lower right-hand corner of a square. If that doesn’t work, I put my second guess in the lower left-hand corner. If that doesn’t work, I give up.

My wife does them with pencil and eraser. I use a ball-point pen and just cross out numbers that have become forbidden. She likes to work on a grid about 0.75 in. square. I find that very hard, and I use a grid that’s 1.2 in. wide by 0.9 in. tall, as big as I can fit on a 8.5- by 11-in. sheet.

She likes to start by putting in all the easy numbers when there are several numbers given. I like to start with the ones and progress right up to the nines. That way, I know that a tiny number between a four and a six must be a five. But that does not explain our differences. We have checked several times, and even though we use different techniques, we come to the same intermediate states. Usually.

GETTING A DO-OVER

Sometimes I find I have made a mistake. I wish I could take
back a lot of my moves, sequentially, back to a certain point
where there are no errors and then start forward again. But
without an infinite amount of record-keeping, you can’t back
up a sudoku game.

Can I get a computer program to let me “back up”? To enable me to go back and see where I made a mistake? And then list all the “possible” numbers and erase them when they become forbidden? Most computers aren’t set up for this. I don’t want a computer that can play the game for me, but I think it would be fun to document the flow. Easier to back up! And I am certainly not going to start writing software for a project as monstrous as this!

Sometimes a two-star puzzle can be quite hard or tricky. Sometimes puzzles that are alleged to be four or five stars can be easy. These “ratings” are quite arbitrary and inconsistent. Beware of some “Latin Squares” that look like a sudoku but may have two or more solutions. I once did a “monster” sudoku that was four by four by (four by four) squares, but I don’t need to do that very often. Maybe if I’m stuck on a desert isle.

Anyhow, it is a nasty time-killer. When I have a short plane trip, I bring several sheets of graph paper so I can do the puzzles from the airline’s in-flight magazine. When I have a long flight, it just ruins my spare time!

And I promise to not bore you about my adventures solving cryptoquips. But these, too, make pretty good mental exercises to keep your mind sharp.