两种方法：

1. 反转图形并计算DFS-loop（）并注意没有出边的顶点（如Abhishek说的那样）。

1. Reverse the graph and calculate DFS-loop() and note the vertices which have no outgoing edges (like Abhishek said).

更高效-使用真，假表在图形上运行DFS-loop（）并跟踪没有传入边的顶点。

More efficient - Run DFS-loop() on the graph and keep track of vertices with no incoming edges using a true, false table.

在最坏的情况下，方法1花费的时间是原来的两倍。

Method 1 takes twice as long in the worst case.