1. School of Mathematics and Statistics, Qinghai Normal University, Xining 810001, China
2. Academy of Plateau, Science and Sustainability, Xining, Qinghai 810008, China
3. Key Laboratory of Tibetan Information Processing, Ministry of Education, Xining, Qinghai Province, China
4. Tibetan Intelligent Information Processing and Machine Translation Key Laboratory, Qinghai 810008, China
5. School of Computer, Qinghai Normal University, Xining 810001, China