1. Center for Combinatorics and LPMC-TJKLC, Nankai University, Tianjin 300071, P. R. China
2. Department of Mathematics, Brock University, St. Catharines, Ontario L2S 3A1, Canada
3. College of Science, Civil Aviation University of China, Tianjin 300300, P. R. China
4. Department of Mathematics, Tianjin Polytechnic University, Tianjin 300387, P. R. China