Hitori · strategies
Shade with certainty.
Shade duplicates so no number repeats in a row or column — shaded cells never touch, and the white cells stay connected. These patterns find the forced moves.
Sandwiched triple
Three of the same number in a row: the middle one stays white. Shading it would force both outer copies white — two duplicates. Shade the outer two.
Pair split by one
Two copies with one cell between (3 · 3): the middle cell is white in every solution — whichever copy survives, the middle can never be shaded next to both. Mark it white and move on.
No adjacent shades
A shaded cell’s four neighbors are white. Every shade you place whitens its neighborhood — and any duplicate among those fresh whites must have its twin shaded. Chains of forced moves start here.
Connectivity (don't cut the board)
All white cells must stay connected. A shade that would fence off a corner or split the grid is illegal — so that cell is white. Watch borders and diagonals of existing shades: diagonal shade chains reaching both edges cut the board.
Order of attack
Triples and split pairs first (they’re free), then propagate around every shade you place, then connectivity checks near the edges to finish.