School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|Division of Biostatistics, School of Public Health, University of Minnesota, Minneapolis, MN 55455, USA. wguan@umn.edu.