# Nurikabe

## Rules

Shade some cells black (leaving the other cells white) so that the grid is divided into non-overlapping regions; cells of the same colour are considered in the same region if they are adjacent horizontally or vertically. Each given number must be in a white region that has the same area in cells as that number. Each white region must have exactly one given number. All black cells must be in the same region. No 2×2 group of cells can be entirely shaded black.

(Rules and example from PGP IB)

## History of the puzzle

First appeared on Nikoli volume 33 (1991). Invented by れーにん ("Renin"). Original title ぬりかべ ("Nurikabe") is a concatenation of ぬり ("to paint") and かべ ("Wall").

## Variants

### Nurikabe Pento

First appeared on the 7th OAPC (2009) under the name "Pentomino Islands". Serkan Yürekli cites Ali Rıza Demirtaş (Turkey) as the developer of the original idea.

Shade some cells black (leaving the other cells white) so that the grid is divided into non-overlapping regions; cells of the same colour are considered in the same region if they are adjacent horizontally or vertically. Each given letter must be in a white region with the shape of pentomino denoted by that letter. Each white region must have exactly one given letter. All black cells must be in the same region. No 2×2 group of cells can be entirely shaded black.

(Example from WPC 2018 IB)

### Tapa Islands

See Tapa#Tapa Islands.

### Word Nurikabe

First appeared on Bram de Laat's (Netherlands) blog in 2011. [1]

Place the given words in the grid, so that the words can be read in horizontally and vertically connected cells. Different words can't touch each other horizontally or vertically. The remaining cells must form a single connected shape and can't have any 2x2 areas anywhere. Each word has one letter given in the grid.

(Rules and example from WPC 2017 IB)

### Full Nurikabe

First appeared on WPC 2016/Round 4 ("Full Classics" round). Author of the puzzle was Matúš Demiger (Slovakia). Note that the solved grid is not a "valid" Nurikabe solution; each white regions can contain more than one numbers.

Shade some cells so that the grid is divided into white regions, each containing the same number of digits as indicated by their value. The area of each white region must be equal to the number it contains. Two white areas may touch each other only diagonally. All shaded cells must be orthogonally connected, but no 2×2 group of cells can be entirely shaded.

(Rules and example from WPC 2016 IB)

### Double Nurikabe

First appeared on WPC 2018/Round 10, "Double Trouble". The puzzle was written by Jiří Hrdina (Czech Rep.).

In the given grid, half of the clues belong to one Nurikabe puzzle and the other half belong to the other Nurikabe. Reconstruct two puzzles and solve them.

(Example from WPC 2018 IB)