1. School of Mathematics and Statistics, Northwestern Polytechnical University, Xi'an, Shaanxi, People's Republic of China
2. Xi'an-Budapest Joint Research Center for Combinatorics, Northwestern Polytechnical University, Xi'an, Shaanxi, People's Republic of China
3. School of Mathematics and Statistics, Qinghai Nationalities University, Xining, Qinghai, People's Republic of China