Hitori ("leave me alone") shades cells until each number is alone in its row and column. No two shaded cells touch; all unshaded cells form one connected region. NP-complete; the playful name is beloved in puzzle communities.
Rules
Grid has numbers with repeats. Shade cells so: no number repeats in any row or column among unshaded cells; no two shaded cells share an edge (diagonal OK); all unshaded cells form one connected region. Solve by logic; unique solution. Adjacent identical numbers: one must be shaded. Shading must not disconnect the unshaded region. The playful name ("leave me alone") refers to each number being left alone in its row and column.
History
Hitori appeared in Nikoli in March 1990. The name means "leave me alone"—each number is left alone in its row and column. NP-complete. The playful name is beloved in puzzle communities. Steady international puzzle book presence; digital apps include Simon Tatham's Portable Puzzle Collection and Nikoli puzzle apps.
Strategy tips
Mark cells you know are unshaded (adjacent to potential shade) first. Adjacent identical numbers: one shaded, one safe. Always check that shading a cell won't disconnect the unshaded region. Use no-adjacency rule to place safe markers.
Cultural context
The playful name is beloved; NP-complete means difficulty scales sharply. Nikoli print and global digital platforms. Hitori ("leave me alone") shades cells until each number is alone in its row and column. The name refers to leaving each number alone. Available on Nikoli.com, Simon Tatham's Portable Puzzle Collection, and various logic puzzle apps. The no-adjacent-shaded and connected-unshaded rules create elegant constraint propagation.
Where to buy / play
- Hitori Puzzle Book (Nikoli) Official Nikoli hitori collection
- Play Hitori free in browser (Simon Tatham) Free, open-source browser implementation