Hitori

Logic puzzle. Shade cells so that no number repeats among the unshaded cells in any row or column. Shaded cells may never touch side by side, and every unshaded cell must stay connected. Solve as many as you can in 150 seconds.

How to play

The board is a grid of numbers. Your job is to shade some cells black so that the remaining clear cells obey three rules. Tap a cell to shade it; tap again to draw a circle on it as a 'keep this one' note to help you think; tap once more to clear it.

The three rules: First, among the clear (unshaded) cells, no number may appear twice in the same row or the same column — duplicates must be shaded away until only one of each remains. Second, two shaded cells may never sit directly next to each other up, down, left, or right (touching at a corner is fine). Third, all the clear cells must form one single connected group — you must be able to walk between any two of them through clear cells.

As you play, a shaded cell turns red if it is illegally touching another shaded cell, and a clear number turns orange while it still has a duplicate in its row or column, so you can spot what is left to fix.

Solve a board and the next one appears instantly. Solve as many as you can before the timer runs out.

Tips & strategy

Hunt for pairs first. Whenever the same number appears twice in a row or column, exactly one of the two must be shaded — and a number that repeats three times in one line means at least two of them get shaded, often pinning down which. These forced duplicates are where almost every solve begins.

Lean hard on the no-touching rule. The moment you shade a cell, all four of its orthogonal neighbours are guaranteed clear, so circle them as 'safe'. If shading a cell would force two shaded squares to touch, then that cell must instead stay clear — which in turn tells you its duplicate partner elsewhere has to be the shaded one.

Watch the corners and edges for connectivity traps. A cell you are about to shade can quietly cut off a clear region or strand a corner; if shading it would leave some clear cells unable to reach the rest, it has to stay clear. Keeping the clear cells as one connected web is often the rule that breaks a tie between two otherwise-legal choices.

When a cell is squeezed between two equal numbers in its row and two in its column, it usually cannot be shaded without isolating something — mark such anchor cells clear early. In the timed mode, chase the forced moves (obvious duplicates and their fences) and only stop to reason when nothing is forced; one shaded cell frequently unlocks a whole corner of the grid.