Nurikabe

Nurikabe is a logic puzzle ("nurikabe" in Japanese folklore is an invisible wall that blocks roads and upon which delays in foot travel are blamed; other names for the puzzle: "Cell Structure", "Islands in the Stream"). The puzzle is played on a grid, typically rectangular with no standard size. Some cells of the grid start containing numbers.

The goal is to determine whether each of the cells of the grid is "black" or "white" according to the following rules:

• All of the black cells must be connected.
• Each numbered cell must be part of a white island of connected white cells.
• Each island must have the same number of white cells as the number it contains (including the numbered cell).
• Two islands may not be connected.
• There cannot be any 2 x 2 blocks of black cells.

Cross+A can solve puzzles from 3 x 3 to 30 x 30.

Line Nurikabe: a grid cannot contain five consecutive black cells in a row or column (2 x 2 cell area can be all black).

Pairs Nurikabe: each island must contain exactly two numbers (instead of one) and have total size equal to the sum of these numbers.