Hitori techniques
11 techniques, all of which the solver can spot for you as a Smart Hint. Each one has a worked example.
Easy
Adjacent Triple
Three equal numbers in a row: only one may stay, and rejected cells can't touch — so the two ends are rejected and the middle is kept.
Sandwich
A cell between two equal numbers is always kept — rejecting it would force both neighbors to stay, and they'd duplicate each other.
Rejected Cell's Neighbors
Rejected cells can never touch, so every cell next to a rejected one is kept.
Duplicate of a Kept Cell
Once a cell is kept, its number can't repeat among kept cells — every other copy in that row and column is rejected.
Don't Trap a Cell
A cell with only one free neighbour left would be sealed off if that neighbour were rejected too — so the neighbour is kept.
Medium
Adjacent Pair
Two equal numbers side by side: exactly one of them stays, so any other copy of that number in the row or column is rejected.
Square of Four
Four equal numbers in a 2×2 square can only be rejected one diagonal or the other. Tucked into a corner of the board, the choice is forced: rejecting the two cells beside the corner would seal the corner cell off, so the corner and its diagonal partner are the rejected ones.
Hard
Don't Split the Board
All kept cells must stay connected — a cell whose rejection would cut the board into two separate regions has to be kept.
Duplicate Trap
Keeping this cell would reject every other copy of its number in its row and column at once — and that burst of rejections would seal part of the board off from the rest. So the cell is rejected instead.