1. School of Computer Science and Technology, Xidian University, 47905 Xian, Shaan xi, China, (e-mail: heli@xidian.edu.cn)
2. School of Computer Science and Technology, Xidian University, Xi'an, Shaanxi, China, (e-mail: jbhuang@xidian.edu.cn)
3. School of Computer Science and Technology, Xidian University, 47905 Xian, Shaanxi, China, (e-mail: h_yuan@stu.xidian.edu.cn)
4. School of Computer Science and Technology, Xidian University, Xi'an, Shaanxi, China, (e-mail: cuijt@xidian.edu.cn)
5. School of Computer Science and Technology, Xidian University, 47905 Xian, Shaanxi, China, 710071 (e-mail: xkma@xidian.edu.cn)
6. School of Software Engineering, Chengdu University of Information Technology, 47909 Chengdu, Sichuan, China, (e-mail: shaojieqiao@gmail.com)
7. Key Laboratory of Knowledge Engineering with Big Data, Hefei University of Technology, 12513 Hefei, Anhui, China, (e-mail: xdwu@gmail.com)