Monster Book of Japanese Puzzles

Hitori Puzzles

Hitori is a Japanese logic puzzle from Nikoli that can be described as a sort of "reverse Sudoku". Whereas with Sudoku you place numbers in a grid, with Hitori you remove them. The name comes from the Japanese for "let me alone" which is a good summary of the puzzle's objective.

Like Sudoku a Hitori puzzle uses the digits 1-9 but is not mathematical in nature: the numbers could be replaced by any unique symbols.

What Is A Hitori?

A Hitori puzzle consists of a grid - usually square - filled with the digits 1-9. The objective is to remove squares by filling them in (blacking them out) so as to leave a situation where the following conditions are met:

There are no duplicate numbers in any row
There are no duplicate numbers in any column
No two black cells may be connected horixontally or vertically
All white cells must form a single group connected by horizontal and vertical movements

Within those apparently simple restrictions Hitori can be extremely difficult to solve. As with Sudoku, it can be difficult to "back up" if a mistake is made. For this reason it's a good idea to take a copy of the puzzle before starting unless using a program with an "undo" feature.

Although Hitori hasn't achieved the popularity of its more famous stable mate it has a loyal following and you can buy Hitori puzzles in books and magazines as well as finding them online.