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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）
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原文地址：http://www.cnblogs.com/sxbjdl/p/5598769.html
