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Nurikabe: Between Islands

A square touching two different islands is wall. As island, it would join them into one island with two numbers.

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This is one of the Nurikabe techniques. Each page shows its technique on the same 8×8 from the game, Medium level 8.

The rule

  1. Look for an empty square with an island on two of its sides, from two different numbers.
  2. Shade it.

In a real puzzle

On the 8×8, it’s first needed at step 3. Row 3, column 3 sits between the 2 to its left and the 2 to its right. Island there would join them, so it’s wall.

Medium level 8, step 3 between-islands: wall row 3, column 3 (from row 3, column 2 & row 3, column 4) 1 2 1 2 2 2 2 2 3 2 2 2 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
Step 3: the square between two 2s is wall.

How often you’ll need it

On 42 of the 50 Easy levels, 49 Medium and all 50 Hard: 1.6 times per Easy level on average, 3.3 on Medium and 4.8 on Hard. It’s 4 of the 42 steps on the 8×8 here.

Next steps

Questions

What about squares diagonal to two numbers?

Those are fine: islands only join side by side. A square has to touch both islands directly to be between them.