Chokepoint
If the only way to reach a cell is through one narrow gap, and going that way traps a 2×2 sea block, the gap must be land.
How it works
- 1Suppose the cell right of the 4 were sea. Together with the two sea cells in the row directly beneath it, that fills three corners of a 2×2 square.
- 2The fourth corner would then be forced to land to avoid a pool — but closing this route leaves the 4 needing four more cells to stretch that far, and the 1 is already finished.
- 3No island could ever claim that corner, so it could be neither land nor sea.
- 4The supposition fails one step before the pool would appear: the cell is land.
- Island (land)
- Sea
- Cells that establish the pattern
- Where the conclusion lands