Hitori is a Japanese logic puzzle published by Nikoli in which players shade selected cells of a number grid until no number appears twice in any row or column of the remaining white cells. Three independent rules govern every valid solution: shading must eliminate all row and column duplicates, no two shaded cells may share an edge, and the unshaded cells must form a single orthogonal connected area. The combination of those three rules turns a duplicate-removal task into a compact spatial logic puzzle. The Lizely implementation is fixed at the standard 5×5 size, and the underlying rules follow the same Nikoli and Simon Tatham Singles definition used by other Hitori solvers. A solution must satisfy all three rules at the same time, and the page's validator checks each one in turn. Failing any single rule triggers a contradiction warning rather than a final defeat, so the player can unshade and continue. This guide explains each rule in plain language, walks through the exact keyboard controls used on the Hitori Puzzle page, and shows how the score and the disclosed fixed solution relate back to the three rules.
The Three Official Hitori Rules
Every Hitori puzzle, regardless of size, is governed by exactly three constraints, and they must hold simultaneously. The validator on the Lizely puzzle checks each one independently and reports a contradiction the moment any of them fails.
Rule 1: Row and column uniqueness. Among the unshaded cells of every row, every number must appear exactly once. Among the unshaded cells of every column, every number must also appear exactly once. When two cells in the same row or column contain the same number, the player must shade at least one of them so that only one copy remains visible. The starting grid is deliberately constructed with duplicate pairs, and solving Rule 1 alone is the entry point to the rest of the puzzle.
Rule 2: No adjacent shaded cells. Two shaded cells may not share an edge. They can touch at a corner, which means diagonal contact is allowed, but they cannot sit directly above, below, left, or right of each other. This rule is the reason many obvious "shade all duplicates" plans fail: shading two duplicates in the same row may force them to touch, and a contradiction appears immediately.
Rule 3: One connected white area. Every unshaded cell must be reachable from every other unshaded cell by stepping only up, down, left, or right through unshaded cells. The white cells together must form one single orthogonal region. Diagonal steps do not count for connectivity. Shading that isolates a white cell, or that carves the white region into two disconnected pieces, is also illegal.
The rules are stated in exactly this form on Nikoli's Hitori page and on the Singles entry of Simon Tatham's Portable Puzzle Collection, which the Lizely implementation references for its validation logic.
Hitori Rules at a Glance
| Rule | Plain meaning | What it forbids |
|---|---|---|
| Row and column uniqueness | Every visible number must be the only copy of that value in its row and in its column. | Leaving two of the same number unshaded in the same row or column. |
| No adjacent shaded cells | Shaded cells cannot share an edge with another shaded cell. | Two shaded squares touching horizontally or vertically (diagonal contact is allowed). |
| One connected white region | All unshaded cells must link through orthogonal neighbors. | Shading that isolates any unshaded cell or splits the white area into two parts. |
The validator checks the three rules in order. It counts visible values separately in all five rows and all five columns and flags any count above one. It then inspects every shaded cell's right and lower neighbors for forbidden edge adjacency. Finally it starts at one white cell and runs a breadth-first traversal over orthogonal white neighbors; if any white cell cannot be reached, Rule 3 has been broken.
How to Play a 5×5 Hitori Grid
The fixed Lizely board is a 5×5 grid of integers. The puzzle starts with deliberate duplicate pairs, and a disclosed tested solution shades five cells. The steps below use the keyboard controls built into the page,
- Move through the grid with the arrow keys. The current cell is highlighted by a visible site-theme outline, and every cell announces its row, column, number, and current state so you always know what you are about to shade.
- Press Space to shade or unshade the selected cell. Each Space press toggles the shaded state. Shading one cell is one move, so use the key deliberately and avoid guessing.
- Press Enter to mark a white-circle note. Notes are optional reasoning markers that flag cells you have proved must remain unshaded. They do not change validation, but they count as moves and reduce the final score by ten points each. Shading a cell automatically removes its note.
- Watch for the contradiction message. If shading two cells forces adjacency, leaves duplicates, or splits the white region, the interface reports a contradiction. Unshade the offending cell and continue rather than restarting the whole puzzle.
- Reach zero duplicates, no adjacent black pair, and full connectivity at once. The board is solved when all three rules hold simultaneously. The score then reflects your move count.
- Let the solved state freeze, or pick Restart. After the puzzle is solved, input freezes so the completed state remains stable. Select Restart to clear shades, notes, moves, score, contradictions, and completion before starting a fresh attempt.
The disclosed solution on the Lizely board shades exactly five cells: the first cell in row one, the center-right cell in row two, the final cell in row three, the second cell in row four, and the fourth cell in row five. Those five shades remove every row and column duplicate, leave no two shaded cells sharing an edge, and keep the white cells as one connected region. Finishing in five moves yields 1,000 − 10 × 5 = 950 points.
When a Hitori Rule Is Broken
Each rule can fail on its own, and the interface reports a generic contradiction warning when any rule is broken. Three common failure modes are worth knowing because they map directly to the three rules above.
- Hidden duplicate. You shaded one copy of a number but missed its twin in the same row or column. Both copies are now white in the same line, and Rule 1 reports a duplicate.
- Adjacent pair. You shaded two cells that sit next to each other horizontally or vertically. The validator flags them as soon as the second is placed, because Rule 2 forbids edge sharing.
- Disconnected white. Your shading has cut the white area into two pieces, so some white cells can no longer reach the others through orthogonal steps. Rule 3 is broken.
The contradiction message is a deadlock warning for the current arrangement, not a permanent game over. The puzzle can always be unblocked by unshading the cell that broke a rule. Restart is only needed if you want a clean slate; otherwise a single Unshade is enough to keep going and preserve your move count.
Score, Moves, and the Restart Path
Score on the Lizely board follows the formula max(0, 1000 − 10 × moves). Every shade counts as one move, and every white-circle note also counts as one move. The disclosed five-shade solution therefore scores 1,000 − 50 = 950 points. Notes are useful for reasoning about cells you have proved must stay white, but each one costs ten points, so a thoughtful note habit can quietly drop you below a clean perfect run.
Gameplay is entirely local. A completed best score may be stored in browser localStorage so you can see whether your next attempt beats your previous run. Nothing is uploaded and no account is required. Restart clears shades, notes, moves, score, contradictions, and completion in one action, which is useful for trying a fresh reasoning line without losing track of old attempts. A separate boss key feature is also available: pressing Escape twice within 400 milliseconds shows a shared simulated operations spreadsheet, and pressing Escape twice again within the same window restores the unchanged puzzle. The cover is visual only and does not alter your work.
Strategy Built from the Three Rules
The three rules are easiest to apply in order. Start with Rule 1 by scanning every row and column for numbers that appear twice. If both copies of a duplicate have no other conflict, one of them can often be shaded immediately. Rule 2 then becomes active: if shading your chosen cell would touch an already shaded neighbor, try the other copy of the duplicate, or look for an adjacent unshaded cell that still satisfies the row or column uniqueness.
Rule 3 usually becomes the deciding constraint only in the last few moves, when shading one more cell risks cutting the white region into two parts. A common safeguard is to leave at least one open path through the middle of the board so connectivity is preserved even after the final shades are placed. Notes are particularly useful during this last phase, because marking cells you have proved must stay white prevents you from accidentally shading them under time pressure.
For a deeper look at the adjacency rule specifically, the article Can Black Cells Touch Diagonally in Hitori? walks through several worked examples of edge versus corner contact.
Related Logic Puzzles with Spatial Constraints
Hitori belongs to a family of deterministic logic boards where every move is constrained by spatial rules rather than chance. Heyawake Puzzle uses a similar shading idea on rooms, with extra white-corridor and three-in-a-row rules layered on top. Nurikabe flips the colors: the shaded cells must form one continuous stream, and the unshaded islands stay isolated around numbered cells. Lights Out shares the appeal of pressing a single key on the selected cell but turns the constraint into a cross toggle on neighbors rather than a duplicate-removal problem. Each game rewards the same habit of reading three rules at once before shading, marking, or flipping a cell.
Related reading: Fewer vs Less Grammar Rule: Practice Countability Safely.