1. Ministry of Education, School of Computer Science, Key Laboratory of Modern Teaching Technology, Shaanxi Normal University, Xi’an, China
2. School of Computer Science, Shaanxi Normal University, Xi’an, China
3. School of Aeronautics and Astronautics, Shanghai Jiao Tong University, Shanghai, China
4. School of Computer Science and Technology, Xidian University, Xi’an, China
5. Department of Computing Science, University of Alberta, Edmonton, Canada