Hitori Puzzle

Hitori is a shading puzzle. Digits fill the grid, and you shade the duplicates so every white digit is unique in its row and column, shaded cells never touch, and the white cells stay connected. You can play grids from 5×5 up to 9×9, and every puzzle here has exactly one solution that you can reach by logic, with no guessing.

Rules

The board is an n×n grid filled with digits from 1 to n. Those digits are given and never change.

Shade some cells black. Among the cells you leave white, each digit appears at most once in every row and every column.

Two shaded cells never share an edge across or down; they may touch at a corner. The white cells form one connected group across and down, never diagonally.

The puzzle is solved when every cell is shaded or left white, no white digit repeats in a row or column, no two shaded cells share an edge, and the white cells are one connected piece.

Controls

Tap a cell to shade it. Drag along a row or a column to shade several cells in one go; the stroke snaps to the line you started in. Tap a shaded cell again to clear it.

Circles mark cells you know must stay white. Pick Circle under the board, and taps and drags place circles instead. With a mouse you don’t need to switch: left-drag uses the pad tool and right-drag uses the other one. The first cell of a stroke decides what it does, so a stroke that starts on a cell that already has that mark clears instead.

On a keyboard, the arrow keys move around the grid. Space, Enter, S or 1 shades the cell, X, C or 2 places a circle, and Backspace, Delete or 0 clears it. Pressing a mark key on a cell that already has that mark clears it.

Two shaded cells that share an edge, two white or circled cells that share a digit in the same row or column, and a white region that can no longer stay connected all turn red at once. Press Hint for one cell the rules already force, with a sentence saying why. Press Check when you want the board checked against the rules. Undo and Redo step through your moves, and Reset clears the board. When the colouring is right, the shaded cells turn teal.

Techniques

Easy grids follow from local digit patterns:

• Sandwiched cell: two equal digits with one cell between them force that middle cell to stay white.
• Triple: three equal digits in a line shade the ends and leave the middle white.
• Adjacent pair: when two equal digits sit next to each other, every other copy of that digit in the same row or column is shaded.
• Corner L: a corner digit that matches both of its neighbours is shaded.
• Last exit: a white cell with only one open neighbour keeps that neighbour white.
• Seen white: once a cell is white, every other copy of its digit in the row or column is shaded, and a shaded cell forces its four neighbours to stay white.

Medium grids also need a short split of an adjacent equal pair:

• Which copy: exactly one of two adjacent equal digits can stay white; trying each side and keeping what both agree on fills the next mark.

Hard grids need a wider view of the white region:

• White-region cut: a cell that would split the white cells if shaded must stay white.
• One-cell try: if shading a cell (or leaving it white) breaks the rules, that cell takes the other mark.

Beginner tips

Start with adjacent equal pairs and triples. An adjacent pair shades every other copy of that digit in the line, and a run of three equal digits shades both ends at once.

Whenever you shade a cell, circle its four neighbours so you do not shade them by mistake. Circles also make the next sandwich or last-exit step easier to see.

If you are new to Hitori, start with the daily 6×6 easy grid, and move up once sandwiches, triples, and adjacent pairs feel natural.