输出一个有向无环图（DAG）的拓扑排序。通过计算各顶点的入度数，删除入度为零的顶点，同时将该顶点指向的点的入度减一。重复此操作直到所有点都被输出。-Output a directed acyclic graph (DAG) topological sort. Through calculating the degree of each vertex deleting zero vertices into degrees, at the same time the vertex point to