School of Computer, Wuhan University, Bayi Road, Wuhan, China.|School of Computer Science and Technology, Heilongjiang University, Xuefu Road, Harbin, China.|School of Computer Science and Technology, Heilongjiang University, Xuefu Road, Harbin, China.|School of Computer, Wuhan University, Bayi Road, Wuhan, China. dhji@whu.edu.cn.