Editorial: Mahjong, Sudoku and Chess

This first issue of 2021 contains two scientific articles. The first one An Efficient AI-based Method to Play the Mahjong Game with the Knowledge and Game-tree Searching Strategy by Mingyan Wang et al. deals with the imperfect information game of Mahjong which has a huge average size of information sets. In order to approach this popular, complex and challenging game, the authors propose to use opponent models, knowledge and tree search.