page_title: Nurikabe (puzzle)
section_title: Advanced strategy
hierarchical_section_title: Nurikabe (puzzle) / Solution methods / Advanced strategy
caption_attribution_description: An image showing the third strategy on the article Nurikabe (puzzle).
context_page_description: Nurikabe is a binary determination puzzle named for Nurikabe, an invisible wall in Japanese folklore that blocks roads and delays foot travel. Nurikabe was apparently invented and named by Nikoli; other names for the puzzle include Cell Structure and Islands in the Stream.
context_section_description: If there is a square consisting of two black cells and two unknown cells, at least one of the two unknown cells must remain white according to the rules.  Thus, if one of those two unknown cells (call it 'A') can only be connected to a numbered square by way of the other one (call it 'B'), then B must necessarily be white (and A may or may not be white).
  If an island of size N already has N-1 white cells identified, and there are only two remaining cells to choose from, and those two cells touch at their corners, then the cell between those two that is on the far side of the island must be black.
  If a square must be white and only two islands can connect to it and have no unidentified cells left after connecting, then if the islands connect at a 90 degree angle (ex: One island can connect to the top side and the other to the right side) the cell inside the angle (The one touching the top-left corner of the white square in the previous example) must be black to avoid connecting the 2 islands.
  Undetermined cells adjacent to a straight row (or a straight column) of black cells can be tested for being black, because if they are black it will form two elbows, and there will be two adjacent white cells which need to be reachable from the islands. If they can not be fulfilled within the constraints, it means the cell that was probed for blackness must be white.
caption: An example of the third advanced strategy The cell diagonally between the two islands has to be black.
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