School of Computer Science and Technology, Xidian University, Xian, 710071, China.|School of Computer Science and Technology, Xidian University, Xian, 710071, China. hwhuo@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, 710071, China.|School of Electronic Engineering, Xidian University, Xian, 710071, China.|Department of Computer and Information Science, The University of Mississippi, Oxford, MS, 38677-1848, USA.|Department of Electrical Engineering and Computer Science, the University of Kansas, Lawrence, KS, 66045, USA.