[Zhao, Yang;Yu, Jiaguo;Zhang, Haofeng] Nanjing Univ Sci & Technol, Sch Comp Sci & Engn, Nanjing 210094, Peoples R China.^[Liao, Shengbin] Huazhong Normal Univ, Natl Engn Res Ctr Learning, Wuhan 430079, Peoples R China.^[Zhang, Zheng] Harbin Inst Technol, Sch Comp Sci & Technol, Shenzhen 518055, Peoples R China.