1. Department of Computing, The Hong Kong Polytechnic University, Hong Kong.
2. Department of Computer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China (e-mail: shengbin@sjtu.edu.cn)
3. School of Computer Science and Engineering, South China University of Technology, Guangzhou 510006, China, also with the Navigation College, Dalian Maritime University, Dalian 116026, China, and also with the Faculty of Science and Technology, University of Macau, Macau 999078, China.