Nikoli Classic • Islands in the Sea
Nurikabe (Islands in the Sea)
Build the archipelago! Shape numbered white islands so each reaches its exact size, while flowing a single connected ocean without any $2\times2$ water pools.
Surveying Nurikabe ocean atoll...
Nurikabe Rules & Objective
Named after the mythological Japanese Nurikabe spirit that blocks paths, this classic Nikoli puzzle requires balancing territory partition with fluid flow constraints:
1. Island CluesEach white island must contain exactly one numbered clue, and its total orthogonal cell count must equal that number.
2. Island IsolationIslands cannot touch each other orthogonally (up, down, left, right). They must be completely separated by sea water.
3. Connected Sea StreamAll dark sea cells must connect into one unbroken stream. No isolated lakes or puddles are permitted.
4. No 2×2 Sea PoolsSea cells can never form a solid 2×2 square anywhere on the board.
Master Deduction Theorems
- Surrounding 1-Islands: Any cell with a clue of “1” is already a complete island by itself. Immediately shade all four adjacent orthogonal neighbors with sea!
- Clues Separated by One Space: If two numbers have only one cell between them, that intermediate cell must be sea water; otherwise, the two islands would merge into one.
- Preventing 2×2 Pools: Whenever you see three sea cells forming an “L” shape in a 2×2 square, the fourth corner must immediately be marked as land (island) to prevent a 2×2 sea violation.
- Sea Escape Bottlenecks: If a sea stream has only one path to escape an enclosed pocket and connect to the rest of the ocean, that path must be extended as sea water.