Hitori: rules, strategy, and free play

THE RULES

1. Shade some cells black. Every cell is either shaded (black) or kept un-shaded. There's no third state.
2. Un-shaded cells: no number repeats within a row or column. If a row originally has two 3s, at least one of them must be shaded. Same for columns.
3. Shaded cells cannot share an edge. Two shaded cells may not be horizontally or vertically adjacent — diagonal contact is fine.
4. Un-shaded cells must form a single connected region. You must be able to walk between any two un-shaded cells via horizontal/vertical steps without crossing a shaded cell. No isolated 'islands'.

BEGINNER STRATEGY

• Sandwich rule (a.k.a. triple). If a number appears three times in a row with the duplicates side-by-side as X-?-X, the middle cell must be UN-shaded (shading any side X would leave the other side still duplicating). Place it as a confirmed un-shaded.
• Pair-with-twin elimination. If two cells in a row both contain the same number AND a third matching number exists elsewhere in that row, BOTH of the adjacent pair stay un-shaded (you can't shade either without orphaning the other) — so the third instance MUST be shaded.
• Adjacent-shaded forces un-shaded. Once you mark a cell shaded, every horizontally + vertically adjacent cell is forced un-shaded by the no-touch rule. Propagate immediately.
• Connectivity check before each shade. Before shading a cell, mentally trace the un-shaded region — if shading creates an isolated island, that shading is illegal. Look for narrow corridors where a single shade would split the grid.
• Work the densest duplicates first. Rows/columns with 3+ copies of a number are the most constrained — each duplicate is a hard decision waiting to be made, and the propagation from those decisions usually unlocks the rest of the board.

COMMON MISTAKES

• Shading too aggressively. When you see a repeated number, the instinct is to shade one immediately. Resist. Check connectivity first — shading might cut the grid into two separate regions, which is illegal.
• Not propagating the no-touch rule. Every time you shade a cell, all four orthogonal neighbors are immediately confirmed un-shaded. Not applying this propagation step leads to missed deductions and wasted scanning later.
• Ignoring the sandwich rule. When the same number appears three times in a row and the two outer appearances are adjacent to the middle one (X-X-X or X-?-X patterns), the middle cell's status is often forced. Always check triple patterns before making harder decisions.
• Checking connectivity only at the end. Connectivity errors compound. Check after every shade — ask: "does shading this cell disconnect any part of the un-shaded region?" A corridor of one cell wide is your warning sign.

HOW TO THINK ABOUT IT

Hitori solves backwards from most number puzzles: you're not building a solution, you're pruning one. The grid starts full. Your job is to find which cells must be removed while keeping the two topology rules intact. Think of the un-shaded region as a single living thing you're trying to keep whole — every shade is an amputation that might leave it in two pieces.

WHY THIS PUZZLE REWARDS YOU

Hitori is GridJoy's puzzle for fans of Nurikabe and other shading puzzles. Unlike Sudoku or Kakuro, you're not placing digits — every digit is already there. Instead, you're carving away the redundancy via shading while preserving global connectivity. The topology constraint (no-touch + single-region) gives Hitori a spatial-reasoning feel that pure number puzzles can't match. It's also famously deceptive: a Hitori grid can look almost-solved with 80% of cells obvious, then collapse when one shading decision in the centre cascades through the whole board.

FORCED DEDUCTION REFERENCE TABLE

These patterns decide a cell's state without guessing. Apply them in order on every pass — propagating a shade (row 3) immediately triggers un-shaded assignments, which in turn may unlock further deductions. Check connectivity (rows 4–5) before committing any shade.

VARIANTS

• Nurikabe. A shading puzzle where you shade cells to form a connected sea of black, leaving numbered islands of exactly the stated sizes. Similar topology rules to Hitori but inverted — you grow black regions instead of avoiding them touching.
• Takuzu (Binairo). A binary placement puzzle with a no-three-in-a-row constraint similar to Hitori's no-duplicate rule. Different structure, but the same habit of 'eliminate by topology' transfers.
• Shikaku. A geometry puzzle where you divide the grid into numbered rectangles. Another pruning-style puzzle — you're carving regions from a whole rather than filling empty cells.

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