Yin Yang

Rules

Fill each cell with either a black or a white circle. All cells with black circles must be connected orthogonally, and all cells with white circles must be connected orthogonally. Every 2×2 group of cells must contain at least one black circle and at least one white circle. Some cells are already filled in for you.

(Rules and example from PGP IB)

History of the puzzle

Invented by Minako Sakai (Japan) in 1993 on the Puzzler magazine. Originally entitled しろまるくろまる (namely "White Circles, Black Circles").

Variants

Outside Yin Yang

Appeared on WPC 2019/Round 2. Author was Jürgen Blume-Nienhaus (Germany).

Divide the grid along the grid lines into two parts, so that within each part all cells must be connected, and each cell must belong to exactly one part. No 2×2 square is located completely within one part. The numbers outside the grid indicate the longest contiguous block of cells in the respective row or column that all belong to the same part.

(Rules and example from WPC 2019 IB. Note that the rules don't specify colour of the region)

Appearances in the past WPCs

• WPC 2019/Round 2 (Outside Yin Yang) by Jürgen Blume-Nienhaus
• WPC 2019/Round 3 (Permaculture hybrid with Masyu) by Silke Berendes
• WPC 2019/World Cup Round 3 by Silke Berendes
• WPC 2017/Round 1 by Rohan Rao
• WPC 2017/Round 13 by Rohan Rao
• WPC 2016/Round 15 (part of a Matchmaker puzzle) by Matej Uher
• WPC 2014/Round 14
• WPC 2013/Part 1