1. College of Information Engineering, Xiangtan University, Xiangtan 411105, P. R. China
2. Department of Computer Science, Guangdong University of Finance, Guangzhou 510521, P. R. China
3. Department of Mathematics and Information Technology, The Hong Kong University of Education, New Territories, Hong Kong
4. Department of Computer Science and Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong