Coral

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Rules

Coral Example.png Coral Example Solution.png

Shade some cells black (leaving the other cells white) such that all shaded cells are connected orthogonally into a single region. All non-shaded cells must be connected orthogonally (through other non-shaded cells) to the edge of the grid. No 2×2 group of cells can be entirely shaded black.

The numbers to the left of (and above) the main grid represent the lengths of contiguous blackened cell blocks in the corresponding row (or column). The lengths are not necessarily given in order from left to right (or top to bottom), and cell blocks must contain at least one unblackened cell between them. As a special case, if the single clue “0” is given, it means there should be no blackened cells in that row (or column).

(Rules and example from PGP IB)

Rule variations

  • In some puzzles, some numbers are replaced with question marks.
  • Coral with Question Marks: a puzzle in WPC 2018/Round 13. Clues are in the correct order. Every clue that are not 2 are replaced with question marks.

History of the puzzle

Invented by Zoltán Horváth (Hungary) in 2001, according to his blog.

Variants

See list of Coral variants.

Appearances in the past WPCs

Listed here are appearances of classic to almost-classic Coral. For variants, see list of Coral variants#Appearances in the past WPCs.