1. Corresponding author. Dongguan University of Technology, School of Computer Science and Technology, Dongguan, Guangdong 523808, China.
2. Institute of Informatics, LMU Munich, 80538 München, Germany.
3. The Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavik, Iceland.
4. Department of Mathematics, University of Sussex, Falmer, BN1 9QH, United Kingdom.
5. Dongguan University of Technology, School of Computer Science and Technology, Dongguan, Guangdong 523808, China.