Hitori

Hitori is a logic puzzle from Japan, introduced by the puzzle publisher Nikoli in the late 20th century. The name Hitori translates to "lonely person" or "one person." The puzzle presents a grid filled with numbers, typically from 1 to 9, which repeat across rows and columns.

The objective is to shade certain cells so that no row or column contains the same number more than once among the unshaded cells. Shaded cells may not touch each other horizontally or vertically, but they may touch diagonally. After shading, the remaining unshaded cells must form a single connected area when traversed through orthogonal adjacency. A valid puzzle has a unique solution.

Solving Hitori is largely a matter of deduction rather than trial and error. For every pair of identical numbers in a row or column, at least one of the two must be shaded. The adjacency constraint and the requirement of connectivity often force further moves, propagating deductions across the grid. Experienced solvers check for possible isolates and ensure that each number appears at most once in every row and column.

Hitori has appeared in puzzle magazines, books, and online platforms, and can be produced in grids of varying sizes, from compact 5x5 layouts to larger 20x20 or bigger. It remains a staple of Nikoli's puzzle lineup and has inspired numerous digital adaptations and puzzle collections worldwide.