All or Nothing
Draw a single closed loop that travels horizontally and vertically through the centres of some white cells. The loop cannot touch or cross itself. Every region must be visited by the loop at most once. When the loop enters a region, it must pass through all the cells of the region before leaving the region. No two unvisited cells belonging to distinct regions can touch each other orthogonally.
(Rules and example from WPC 2016 IB. Shading in the solution is for illustrative purposes.)
History of the puzzle
Invented by Naoki Inaba (Japan) in 2003 under the name オールorナッシング (partial transliteration of the words "all or nothing"). First appeared on his website.
Appearances in the past WPCs
- WPC 2016/Round 8 by Matej Uher