1. Department of Mathematics, Minjiang University, Fuzhou 350108, China
2. Center for Discrete Mathematics and Theoretical Computer Science, Fuzhou University, Fuzhou 350108, China
3. School of Computational and Applied Mathematics, Faculty of Science, University of the Witwatersrand, (Wits), Johannesburg 2050, South Africa
4. TCSE, Faculty of Engineering and Build Environment, University of the Witwatersrand, (Wits), Johannesburg 2050, South Africa