1. School of Computer Science and Engineering, Beihang University, Beijing, China
2. Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong
3. Department of Computer Science and Engineering, The Hong Kong University of Science and Technology, Sai Kung, Hong Kong
4. School of Software, Beihang University, Beijing, China
5. School of Computing, University of Leeds, Leeds, U.K