（1）采用邻接表作为有向图的存储结构；

（2）给出所有可能的拓扑序列。

（3）测试用例见严蔚敏《数据结构习题集(C语言版)》p48题7.9图

-(1) adjacency list used as the storage structure of directed graphs　(2) gives the sequence of all possible topologies. (3) test cases, see Yan Wei Min " data structures problem sets (C language version)"　p48 Figure 7.9 title