  We prove NP-completeness of Yin-Yang / Shiromaru-Kuromaru pencil-and-paper
puzzles. Viewed as a graph partitioning problem, we prove NP-completeness of
partitioning a rectangular grid graph into two induced trees (normal Yin-Yang),
or into two induced connected subgraphs (Yin-Yang without $2 \times 2$ rule),
subject to some vertices being pre-assigned to a specific tree/subgraph.
