Department of Computer Science and Technology Xian Jiaotong University, West Xianning Road, 710049 Xian, P,R, China. baojp@mail.xjtu.edu.cn.|Department of Computer Science and Technology Xian Jiaotong University, West Xianning Road, 710049 Xian, P,R, China. baojp@mail.xjtu.edu.cn.|Department of Computer Science and Technology Xian Jiaotong University, West Xianning Road, 710049 Xian, P,R, China. baojp@mail.xjtu.edu.cn.