1. School of Computer, Qinghai Normal University, Xining 810008, China
2. The State Key Laboratory of Tibetan Intelligent Information Processing and Application, School of Computer, Xining 810008, China
3. Academy of Plateau Science and Sustainability, Qinghai Normal University, Xining 810016, China
4. School of Mathematics and Statistics, Qinghai Normal University, Xining 810008, China