1. School of Mathematics and Computational Science, Xiangtan University, Xiangtan 411105, Hunan, P. R. China
2. School of Computer Science, Xiangtan University, Xiangtan 411105, Hunan, P. R. China
3. Key Laboratory of Intelligent Computing and Information Processing, of Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan, P. R. China