1. Department of Computer Science and Technology, Tsinghua University, Beijing, China
2. Department of Computer Science and Technology, Tsinghua University, Beijing, China and Key Laboratory of Pervasive Computing, Ministry of Education, Beijing, China
3. School of Software, Tsinghua University, Beijing, China
4. University of California, Los Angeles, Los Angeles, CA, USA
5. Department of Computer Science and Technology, Tsinghua University, Beijing, China and Qinghai University, Xining, China