A Nikoli classic since 1990
Hitori first appeared in 1990 in issue #29 of Puzzle Communication Nikoli, the Japanese magazine that also made Sudoku a household name. Its full title, 'Hitori ni shite kure', translates to something like 'leave me alone' — a joke about the goal: you eliminate numbers until every one that remains is the only copy of itself in its row and column.
The genre has travelled well beyond the magazine. It shipped on the Nintendo DS as Puzzle Series Vol. 10: Hitori, appears in Simon Tatham's Portable Puzzle Collection under the name Singles, and even featured in episode 11 of the anime xxxHOLiC. The rules have not changed once in three decades — a good sign for a logic puzzle.
The three rules
Shade cells so that: (1) no number appears more than once among the unshaded cells of any row or column — shaded duplicates are fine; (2) no two shaded cells are horizontally or vertically adjacent, though corners may touch; (3) the unshaded cells all stay connected as one region.
Each rule alone is trivial. Together they interlock: a shade that fixes a duplicate can create an illegal adjacency, and a shade that is legal by the numbers can still wall off a corner of the board. Reading those interactions is the whole game.
Starter techniques: sandwich, triple, and pair
The sandwich (X-Y-X): when two equal numbers sit two apart with one cell between them, the middle cell is always white. If it were shaded, both of its equal neighbors would have to stay white — a duplicate. Circle it and move on; this single pattern opens more boards than any other.
The triple (X-X-X): three equal numbers in a row can keep at most one unshaded, and shaded cells cannot touch — so the only legal arrangement is shaded-white-shaded. Both ends get shaded immediately, no further thought required.
The pair rule: when two equal numbers are adjacent, exactly one of them will end up shaded — which means every OTHER copy of that number in the line is a duplicate of the surviving white one, and must be shaded.
Add the starts — any number with no duplicate in its row or column can never legally be shaded, so it is white from the outset — and you have the full easy-tier toolkit.
The cascade: let each move pay for the next
Every deduction in Hitori triggers more. Shade a cell and its four neighbors are guaranteed white — circle them. Circle a cell and every other copy of its number in that row and column becomes a duplicate of a confirmed white — shade them. Those shades circle new neighbors, which shade new duplicates, and a single confirmed cell can ripple across half the board.
Disciplined solvers work the cascade to exhaustion before hunting for new patterns. On this site the auto-circle assist handles the first half of the loop for you, and the technique is why our easy grade never needs anything else: easy boards are exactly the ones the cascade can carry end to end.
Intermediate patterns: corners and double-pairs
Corner patterns are one-cell proofs by contradiction. In a 2×2 corner where the corner cell's number duplicates both of its neighbors, assuming the corner is white forces both neighbors shaded — sealing the corner off from the rest of the board and violating connectivity. So the corner is shaded. Several corner shapes follow this template: assume, cascade one step, contradict.
The 2×2 double-pair works the same way: two overlapping pairs in a 2×2 block leave only one legal shading once you test each candidate against the adjacency rule. Medium-grade boards on this site are precisely the ones that need these local eliminations — you never have to look beyond a small neighborhood to break them.
Advanced play: connectivity and wall arguments
The third rule is the expert's weapon. Shaded cells slowly build walls, and any shade that would complete a wall — cutting the unshaded cells into two islands — is illegal, which proves that cell white. Conversely, when a white region's only remaining exit runs through a single cell, that cell must stay white no matter what the numbers say.
Hard boards on this site are graded hard exactly because somewhere in the solve the duplicates go quiet and only a connectivity argument moves the board forward. And a guarantee worth repeating: every puzzle here is verified to have one unique solution reachable by these techniques alone. Boards that would force guess-and-restore bifurcation are thrown away at generation.