Highlander
Extreme
Used by Smart Hint
Uses the fact that the puzzle has exactly ONE solution: any move that would leave two different valid answers must be wrong. Powerful, but it's reasoning about the puzzle rather than about the loop.
- Cells that establish the pattern
How it works
- Every puzzle here is built to have exactly one solution. That's a fact about the puzzle you're allowed to use, and it can settle positions no amount of loop logic will.
- The shape to look for: a small patch of board where two different arrangements would both satisfy every clue touching it, and would look identical to the rest of the board from outside — same edges crossing the patch's border, same everything.
- If such a pair existed in a real solution you could swap one for the other and get a second valid answer. The puzzle promises there isn't one, so any move that leaves both arrangements open must be wrong — kill it.
- A 2 with no clued neighbours is the classic seed: its two ways of turning are often interchangeable from the outside, so whatever breaks the tie is forced.
- Worth knowing, and it will save you time on a hard board — but treat it as a last resort. It tells you nothing about WHY the loop goes that way, and it's the one technique that stops working the moment a puzzle has a second solution by mistake.