若在DGA图中存在一个顶点r，在r和图中所有其他顶点之间均存在由r出发的有向路径，则称该DAG图有根，试编写求DAG图的根的算法。 -If there is a DGA map vertex r, in r and the graph all the other vertices exist in between the departure from the r to the path are said to have roots in the DAG plans, test plans p