1. Hunan Provincial Key Laboratory of Intelligent Computing and Language Information Processing, Hunan Normal University, Changsha, China
2. School of Information Science and Engineering, Hunan Normal University, Changsha, China
3. Department of Computer Science, Brunel University London, Uxbridge, U.K.
4. Xiangjiang Laboratory, Hunan University of Technology and Business, Changsha, China
5. School of Computer Science and Electronic Engineering, University of Essex, Colchester, U.K.