1. School of Computer Science and Technology, Anhui University, Hefei 230601, Anhui, China. suyansen1985@163.com
2. School of Automation, Huazhong University of Science and Technology, Wuhan 430074, Hubei, China. {tfwu, fei_xu}@hust.edu.cn
3. Department of Computer Science, University of Bucharest, Str. Academiei nr.14, sector 1, C.P. 010014, Bucureşti, România. apaun@fmi.unibuc.ro