What are the rules of Hitori?
Hitori starts with a grid full of numbers, some repeated. Shade squares until no number appears twice among the unshaded squares of any row or column. Two shaded squares may never touch side to side, and the unshaded squares must stay joined up in one piece. Shade only to remove a repeat: every shaded number appears again, unshaded, in its row or column.
Choose a 5 × 5, 6 × 6 or 8 × 8 grid (6 × 6 is picked to begin with); about 30% of the squares end up shaded. Tap a square once to shade it, twice to circle a number you are keeping, and again to clear it. On a keyboard, the arrows move, S shades, C circles and Backspace clears. Shaded squares that touch, and two circled equal numbers in a line, are outlined at once.
How do you solve Hitori?
- Circle around every shaded square. Its neighbours can't be shaded, so they are kept.
- Look for a sandwich. In A B A along a line, one of the As must go, and shading B would block both of them, so B is kept.
- Look for a pair. Of two equal numbers side by side, one is shaded and one is kept, so the same number anywhere else in that line must be shaded.
- Never cut the board apart. If shading a square would wall off part of the unshaded squares, it stays unshaded. This is often the last step that settles a corner.
How is your Hitori Puzzle time calculated?
Your result is the time from Start until the board keeps every rule; circles are only notes, and a square you leave unmarked counts as unshaded. Beside the time are the times you used Check, which marks any wrong shading or circle, and Hint, which fixes a wrong mark or shades a square that must be shaded. There is no rating, because no published typical time exists for puzzles built like these.
Does every Hitori here have exactly one answer?
Yes. Each puzzle is made in your browser when you press Start. It begins as a grid with no repeats at all; some squares, never touching and never cutting the rest apart, are given a number that repeats one already in their row or column. The puzzle is kept only if a solver using the moves above (plus looking one step ahead) can finish it, and a separate search finds exactly one answer. The site's tests make puzzles of every size and check both again.
Where does Hitori come from?
Hitori is one of the puzzles published by Nikoli, the Japanese puzzle company, and its name means “alone”: the numbers left are each alone in their row and column. Most Hitori puzzles never state the “shade only repeats” rule because they are made to have one answer; it is spelled out here so that you never have to wonder. Nurikabe is another shading puzzle with a joined-up rule, and Binary Puzzle is about balancing a line rather than clearing it.