1. College of Mathematics and Systems Science, Shandong University of Science and Technology, Qingdao, Shandong
266590, China
2. Department of Computer Science, Middlesex University, London, NW4 4BT, UK
3. College of Computer
Science and Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China
4. College of Computer Science and Technology, China University of Petroleum, Qingdao, Shandong 266580, China