（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 structure