3.2 Basic graph types基本的图类型
Graph – Undirected graphs with self loops(有自环的无向图)
DiGraph - Directed graphs with self loops(有自环的有向图)
MultiGraph - Undirected graphs with self loops and parallel edges(有自环的无向多重图)
MultiDiGraph - Directed graphs with self loops and parallel edges(有自环的有向多重图)