1. School of Computer Science and Technology, Soochow University, 1 Shizi Street, Suzhou 215006, China
2. College of Mathematics and Computer Science, Fuzhou University, 2 Xueyuan Road, Fuzhou 350108, China
3. Jiangsu High Technology Research Key Laboratory for Wireless Sensor Networks, 66 Xinmofan Road, Nanjing 210003, China
4. Department of Computer Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong