Chao Yang’s research while affiliated with Sun Yat-Sen University and other places

We show that the HANANO Puzzle, a side-viewed 2-dimensional combinatorial puzzle with gravity and colored blocks, is NP-hard.

Lin-Lu-Yau introduced an interesting notion of Ricci curvature for graphs and obtained a complete characterization for all Ricci-flat graphs with girth at least five [1]. In this paper, we propose a concrete approach to construct an infinite family of distinct Ricci-flat graphs of girth four with edge-disjoint 4-cycles and completely characterize all Ricci-flat graphs of girth four with vertex-disjoint 4-cycles.

Sokoban is one of the most studied combinatorial puzzle game in the literature. Its computational complexity was first shown to be PSPACE-complete in 1997. In 2005, a new proof of this result was obtained by Hearn and Demaine, by introducing the Nondeterministic Constraint Logic (Ncl) problem [R. A. Hearn and E. D. Demaine, PSPACE-completeness of sliding-block puzzles and other problems through the nondeterministic constraint logic model of computation, Theoretical Computer Science 343 (2005) 72-96]. Since then, Ncl has been used to prove the PSPACE-completeness of several other puzzles including a few Sokoban variants, by many authors. In this paper, we show that Snowman, a new Sokoban-like puzzle game released in 2015, is PSPACE-complete by reduction from Ncl.