I discovered this distant cousin of Sudoku in the newsgroup rec.puzzles a couple of weeks ago. Kevin Stone has provided an enhanced user interface with notes for doing daily Hitori puzzles on his website brainbashers.com. His site provides 3 sizes: 5×5, 7×7 and 9×9. These puzzles change daily. Previous Puzzles revert to Neil Berget’s beta website hitoriconquest.com. An archive is not yet available. Hitoriconquest.com offers 5×5, 8×8 and 12×12 size puzzles, as well as a tutorial on how to play.
Hitori is Japanese for “Let Me Alone”. More information is in the Wikipedia Article. The Nicoli Company Website has an animated (flash) tutorial.
The rules are straightforward. Shaded cells cannot be contiguous. They are either separated by a non-shaded (‘white’) cell or touch at their corners. Non-shaded cells, on the other hand, must be contiguous (e.g. at least one side must touch another ‘white’ cell’s side.) The result is that when a puzzle is solved, one can travel a connected path of ‘white’ cells. Puzzles provided by these web sites lead logically to unique solutions.
Hitori exercises your brain differently than in Sudoku. It’s almost as if your luggage is too full to close and you must toss expendable articles so that it fits. I found it necessary to go down each row, making the numbers unique, then going across each column. When this is done, I checked that I didn’t isolate any ‘white’ cells. That seemed to be sufficient to form a correct answer.
Both websites have a (partial) solution checker and it can show any mistakes that are evident as well as if it is all correct. I definitely enjoy this variation on a Sudoku board.