1. Key Laboratory of Pervasive Computing, Ministry of Education, Department of Computer Science and Technology, Tsinghua University, Haidian Qu, Beijing Shi, China
2. Key Laboratory of Pervasive Computing, Ministry of Education, Department of Computer Science and Technology, Tsinghua University, Haidian Qu, Beijing Shi, China and Intelligent Computing and Application Laboratory of Qinghai Province, Qinghai University, Xining, Qinghai, China
3. Tsinghua University, Haidian Qu, Beijing Shi, China
4. Microsoft Research Asia, Haidian Qu, Beijing Shi, China and SKLSDE Lab, Beihang University, Haidian Qu, Beijing Shi, China
5. Beijing University of Posts and Telecommunications, Haidian Qu, Beijing Shi, China
6. Microsoft Research Asia, Haidian Qu, Beijing Shi, China
7. Massachusetts Institute of Technology, Cambridge, Massachusetts, United States