Department of Computer Science, Hong Kong Baptist University, Kowloon Tong, Hong Kong. chxw@comp.hkbu.edu.hk.|Department of Computer Science, Hong Kong Baptist University, Kowloon Tong, Hong Kong. chxw@comp.hkbu.edu.hk.|Department of Computer Science, Hong Kong Baptist University, Kowloon Tong, Hong Kong. chxw@comp.hkbu.edu.hk.|Department of Computer Science, Informatics Institute, Christopher S, Bond Life Science Center, University of Missouri, Columbia, MO 65211, USA. chengji@missouri.edu.