direetod graph的用法和样例：
例句
- And the coefficients of eigen-polynomial expressions were closely related to the circular sets of a directional graph.
而特征多项式的系数与有向图的圆族有着密切的关系。 - In a directed graph, the decision variables of the shortest path computation must be determined by a constraint that is called Kirchoff's constraint.
摘要已知一个有向图，求解两点间的最短路径的决策变量满足基尔霍夫约束关系。
