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