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Nurikabe: A Wall With One Way Out

The wall has to be one piece. A piece of wall with only one way out, while there’s wall elsewhere, must go that way to join up.

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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. Find a piece of wall that isn’t joined to the rest yet.
  2. List the empty squares touching it.
  3. If there’s only one, shade it.

In a real puzzle

On the 8×8, it’s first needed at step 22. The wall in rows 3 and 4 of column 1 is closed in: the 1 sits above it, and the 2 and the square it just grew into sit to its right. To join the rest of the wall, it has to go down into row 5.

Medium level 8, step 22 wall-one-way: wall row 5, column 1 (from row 3, column 1 & row 4, column 1) 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 22: the wall in column 1 can only join up by going down.

How often you’ll need it

On 29 of the 50 Easy levels, 34 Medium and 41 Hard: 0.8 times per Easy level on average, 1.2 on Medium and 1.3 on Hard. It’s 2 of the 42 steps on the 8×8 here.

Next steps

Questions

What if a piece of wall has no way out at all?

Then something is wrong: the wall can never join up. Undo back to the last move you were sure of.