Hitori Techniques

Six techniques solve every Hitori on PeerPuzzles. Two of them, shaded neighbours and repeats, make a chain that does most of the work; sandwiches and pairs start it; cut-off and what-if take over when it stops. Each page shows its technique on the same 9×9 puzzle from the game, at the moment it’s first needed, so you can follow one solve from page to page.

Hard level 5, the 9×9 every technique page follows. It takes 55 steps and needs all six techniques.

How many of the game’s levels need each one, out of 50 in each track:

Technique	Easy 5×5	Medium 7×7	Hard 9×9
Shaded neighbours	50	50	50
Repeats	50	50	50
Sandwich	50	50	50
Pairs	9	12	14
Cut-off	48	50	50
What if	0	7	50

“Needs” means the hints, which always use the simplest technique that works, used it at least once. For all six in one place, with another worked puzzle, see how to solve Hitori.

Guides

Hitori Sandwich: A Number Between Two of the SameA number between two of the same in a Hitori line, like 4 2 4, always stays unshaded. Why, and how to use it to start any puzzle, with a real example.
Hitori Repeats: Shade a Kept Number's TwinsOnce a Hitori number is sure to stay unshaded, every other copy in its row and column must be shaded. With a real example.
Hitori Shaded Neighbours: Circle Around Every ShadeShaded Hitori squares can't touch side by side, so every square beside a shaded one stays unshaded. With a real example.
Hitori Pairs: Two Alike Side by SideWhen two of the same Hitori number sit side by side, one of them stays unshaded, so any third copy in that line must be shaded. With a real example.
Hitori Cut-Off: Keep the Grid Joined UpAll the unshaded squares in a Hitori must join up. A square whose shading would wall something in or cut the grid in two stays unshaded. With a real example.
Hitori: What If? Follow a Choice to a ContradictionWhen no simpler Hitori technique works, suppose a square is shaded or kept, follow what it forces, and see whether a rule breaks. With a real example.