Hitori

Hitori is a type of shading logic puzzle, in which solvers must shade squares such that no two identical cells in a row or column remains unshaded. Additionally all unshaded cells must form a contiguous group, and shaded cells may not be adjacent to each other.

Background

Hitori was first introduced in Puzzle Communication Nikoli #29 in 1990, invented by たけゆたか ("Takeyutaka"). The original name for the genre was ひとりにしてくれ ("Hitori ni shitekure") which means "leave me alone". This was later shortened to Hitori, which is just the "alone" part. The constraint of shaded cells being nonadjacent and unshaded cells being contiguous is sometimes known as a "dynasty" constraint. This constraint is inspired by Japanese crossword conventions.

Puzzle Application

An example Hitori puzzle

The solution to the above puzzle

Hitori is a relatively flexible type in that numbers do not really have any meaning beyond being symbols to be matched. Indeed, the numbers can be replaced with letters to create an equivalent puzzle.

Since every cell is initially filled with a number, it makes Hitori a fairly scanning-heavy genre for the grid size. To mitigate this, numbers that are not involved in the logic can be omitted from the puzzle. Interfaces such as puzz.link can highlight numbers that have an identical unshaded copy in its row or column red, drastically reducing the amount of scanning required.

Strategy

• After shading a cell, you may be able to propagate additional deductions by looking at all adjacent cells which must be unshaded, and therefore any identical copies in their respective rows or columns must be shaded.
• If two cells are adjacent and identical, then one of these cells must be shaded. This also means that if there are any other identical cells in the same row or column as the two cells, those cells must be shaded as well (effectively a pointing pair).
• If two cells are identical and are spaced one apart, then the cell between them must be unshaded (because at least one of the two cells must be shaded).
• Connectivity plays a fairly big role in the solving process, especially if large "rooms" with many alternating shaded and unshaded cells appear. It also may be helpful to look at the corners or the sides as well.
• If a cell has no remaining identical unshaded cells in its row or column, it can be left unshaded if the puzzle is unique. (However, note that uniqueness usually can't be assumed in hunt puzzles!) However, this is unlikely to provide any additional information in solving the puzzle, though it may help identify where a deduction could be made.

Examples

Witness 1 Meta (MH 2008) (web) - A metapuzzle where the feeders are arranged in a grid and solved as a Hitori. TO DO

See Also

• List of Hitori puzzles