# Hitori

## Rules

Remove some numbers from the grid so that all remaining numbers are connected orthogonally and no two removed numbers are adjacent orthogonally. Additionally, for each row and each column, the remaining numbers must be all different.

(Rules and example from PGP IB)

### Rule variations

• There are no reasons for a Hitori puzzle to use numbers; some puzzles use letters. (example: WPC 2016/Round 12, PGP 2014 Round 2)

## History of the puzzle

First appeared on Nikoli volume 29 (1990). Invented by たけゆたか ("Takeyutaka"). Original title ひとりにしてくれ ("Hitori ni shitekure") means "leave me alone." "Hitori" corresponds to "alone."

## Variants

### Strong Hitori

First appeared on 123 GO, a contest held in September 2012.[1] The author of the puzzle was Rohan Rao (India). This is Hitori with a weaker shading rule.

Shade some of the digits in the grid so that each row and each column contains distinct digits. All white cells must form a single connected area. Every row and every column must contain at least one white cell.

(Rules and example from WPC 2017 IB)

### Regional Hitori

Invented by Deb Mohanty (India) in 2014 for a planned LMI contest, but was never used. Later appeared on WPC 2017/Round 4.

Shade some of the digits in the grid so that each row, each column and each thickly outlined region contains distinct digits. Black cells can’t touch each other by a side. All white cells must form a single connected area.

(Rules and example from WPC 2017 IB)

### Nonconsecutive Hitori

First appeared on WPC 2018/Round 5. The author of the puzzle was Petr Lichý (Czech Rep).

Shade some numbers from the grid so that all remaining numbers are connected orthogonally and no two removed numbers are adjacent orthogonally, so that no consecutive numbers lie next to each other. (Numbers are allowed to repeat in rows/columns!)

(Example from WPC 2018 IB)