What is Hitori?
Hitori is played on a grid of squares. At the beginning, each cell contains a
number. The goal is to blacken some cells so that there are no white duplicate
numbers in any row or column, similar to the solved state of a Sudoku puzzle
(except with black squares added to the grid). Orthogonal connections are
important as well; painted-out (black) cells cannot be connected orthogonally,
and the other cells must be connected orthogonally in a single group. (i.e. no
two black squares can be adjacent to each other, and all un-painted squares
must be connected, horizontally or vertically, to create a single shape.)
How to Play
You can navigate the grid using the arrow keys or clicking on the square
using the mouse. Clicking on a square will select it, shading the square. To
flip the square to black, you must press the spacebar. To flip the square back
to white, just press the spacebar again.
Solving techniques
An easy starting place is a sequence of three identical numbers; the centre
number must be white as if it was black it would have to be next to another
black square; both ends must be painted out to satisfy the rules.
When it is confirmed that a cell must be black, one can see all adjacent cells
must not be black. Some players find it useful to circle any numbers which must
be white as it makes the puzzle easier to read as you progress.
When two identical numbers are adjacent and the row or column also includes
another identical number then the single number must be black - leaving it
white would result in the two adjacent numbers being black, which is not
permitted.
Any cells which if painted would segregate a group of circled white numbers
from the rest of the puzzle must be white and can therefore be circled.
Any number which is on the same row or column as an identical circled white
number must be black. Any number that is adjacent to two identical numbers on
opposite sides of the cell must be white as one of the adjacent numbers has to
be black and cannot be adjacent to another black cell.
When four identical numbers are in a two by two square on the grid there must
be two black cells along a diagonal. There are two possible combinations, and
it is sometimes possible to decide which is correct by considering if one or
the other variations will cut white squares off from the remainder of the grid.