Department of Computer Science and Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong. lylo@cse.cuhk.edu.hk.|Department of Computing and Decision Sciences, Lingnan University, Tuen Mun, Hong Kong. mlwong@ln.edu.hk.|Department of Computer Science and Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong. leekinhong@hotmail.com.|Department of Computer Science and Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong. ksleung@cse.cuhk.edu.hk.