1. School of Cyberspace Science and Technology and the School of Computer Science, Beijing Institute of Technology, Beijing, China
2. School of Computer Science, Beijing Institute of Technology, Beijing, China
3. School of Cyberspace Science and Technology, Beijing Institute of Technology, Beijing, China
4. School of Computer Science, Beijing University of Technology, Beijing, China
5. Division of Natural Science and Mathematics, Oxford College of Emory University, Oxford, GA, USA
6. Department of Computer and Information Sciences, Temple University, Philadelphia, PA, USA