Department of Computer Science, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China. guoxian85@gmail.com.|Department of Computer Science, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China. hlzhu@comp.hkbu.edu.hk.|Department of Computer Science, George Mason University, Fairfax, VA, USA. carlotta@cs.gmu.edu.