1. College of Mathematics and Computer Science, Performance Computing and Stochastic Information, Processing (Ministry of Education of China), Hunan Normal University, Changsha 410081, P. R. China
2. College of Computer Science and Electronic Engineering, Hunan University, Changsha 410081, P. R. China
3. Nanjing University of Information Science & Technology, Nanjing 210044, P. R. China