Singles / Hitori
Why does removing duplicate numbers not finish Hitori?
The remaining numbers must be unique in every row and column, black cells must not share edges, and all white cells must remain one connected region. Meeting only the duplicate rule is insufficient.
Worked example
Blackening a cell that separates two white areas can invalidate the board even when it removes the last duplicate.
What to check on your board
Apply the rule above to your current board, then verify the remaining puzzle constraints. The example explains a local deduction; it is not a solution to every randomly generated board.
Checking your next move
After removing a duplicate, immediately check the black cell’s edge neighbours and the remaining white connections. A locally useful removal can cut a narrow connection between white areas. Finish with three passes: row and column uniqueness, black-cell adjacency, then white connectivity.
Using this reference
For Singles / Hitori, read the rule and the worked example together before opening the game. Separate what the rule explicitly requires from what only seems likely on the current board. A useful next move is one you can explain from the visible constraints, not merely a pattern that looks familiar.
If you get stuck, isolate a smaller part of the position and list what is still possible there. After a move, check the affected rows, regions or connections again. The reference explains the documented rules; progress, controls and save behavior belong to the linked game.
Read the developer’s manual ↗ · Open the developer’s browser version ↗
Manual inspected: · Simon Tatham & contributors · MIT