1. College of Information Science and Engineering, Hunan Provincial Key Laboratory of Intelligent, Computing and Language Information Processing, Hunan Normal University, Changsha, P. R. China
2. Hunan Xiangjiang Artificial Intelligence, Academy, Changsha, P. R. China
3. Information Technology Institute of China, Railway Guangzhou Group Co., Ltd., P. R. China
4. School of Computer Science and Engineering, Central South University, Changsha, P. R. China