1. School of Computer Technology and Application, Qinghai University, Xining 810016, China
2. Intelligent Computing and Application Laboratory of Qinghai Province, Qinghai University, Xining 810016, China
3. School of Computer and Information Technology, Beijing Jiaotong University, Beijing 100044, China
4. Department of Computer Science and Technology, Tsinghua University, Beijing 100084, China