DirectedGraphQ[g] 如果图 g 是一个有向图，则产生 True；否则产生 False.
检验一个图是否是有向图：
DirectedGraphQ 对于非有向图给出 False：
UndirectedGraphQ DirectedEdge DirectedEdges Graph
400+ new features, including the new Wolfram Predictive Interface, social network analysis, enterprise CDF deployment, and more »
