Nurikabe techniques

Nurikabe rewards patience at the start. A few easy patterns split the board into wall and island, and each one you settle makes the next easier to see. Begin with the small numbers.

In each picture, the cells the technique decides are outlined in blue.

1. Wall in the 1s

   A 1 is an island all by itself, so every cell beside it is wall. Shade all four straight away.

2. Cells touching two numbers

   Two numbers can never share an island. A blank cell next to two different numbers would join them, so it must be wall.

3. Out of reach

   Every unshaded cell belongs to some number’s island. If no island could stretch far enough to reach a cell, it’s wall. Here the 2s can only reach their neighbors, so the middle is wall.

4. No 2×2 of wall

   The wall can’t fill a 2×2 block. When three cells of a square are shaded, the fourth stays unshaded. Mark it with a dot, then work out which island it belongs to.

5. An island with one way to grow

   An island that still needs cells and has only one free side must grow that way. The 3 here is blocked on the right, so it has to grow down.

6. Keep the wall in one piece

   All the wall has to connect. A piece of wall with only one way out must extend that way, or it would be cut off from the rest.

When you’re stuck, count. Add up the numbers to see how many cells are island, and look for a spot where the wall would have to pass through a bottleneck. Then ask what would happen if a cell were shaded: a 2×2 block or a cut-off wall means it can’t be.