1. Department of Information and Computing Science, College of Science; Central South University of Forestry and Technology; Changsha 410004 Hunan P. R. China
2. Key Laboratory of High Performance Computing and Stochastic Information Processing (HPCSIP) (Ministry of Education of China), Department of Information and Computing Science, College of Mathematics and Computer Science, Hunan Normal University; Changsha 410081 Hunan P. R. China