# Hitori

## Rules[edit]

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)

## History of the puzzle[edit]

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[edit]

### Strong Hitori[edit]

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[edit]

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[edit]

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

Remove 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)

## Appearances in the past WPCs[edit]

- WPC 2019/Round 9 by Ulrich Voigt
- WPC 2018/Round 1 by Jiři Hrdina
- WPC 2018/Round 5 (Nonconsecutive Hitori) by Petr Lichý
- WPC 2018/Individual Playoffs (unused) by Petr Lichý
- WPC 2017/Round 4 (Regional Hitori) by Deb Mohanty
- WPC 2017/Round 4 (Strong Hitori) by Rohan Rao
- WPC 2016/Round 12
- WPC 2016/Round 13 (Irregular grid)
- WPC 2015/Round 2
- WPC 2014/Round 3
- WPC 2012/Part 5
- WPC 2010/Part 2