We express our appreciation to Professor Changqing Guo for his valuable suggestions for this study. We also sincerely thank Professors Feng Jin, Jinting Wang and Yanjie Xu for their discussions with us. We acknowledge the assistance of Mr. Tao Li, Mr. Kui Zhang, and Ms. Shanshan Guo as well. Moreover, this work was supported by the National Natural Science Foundation of China (Grant Nos. 51479098 , 51239006 and 91215301 ).