Topological sorting
Pictured first asked DAG
Algorithm:
First, a node of degree 1 is added to the queue
in each case taking the first team point u, and u omitted in the figure adjacent edges
continue to point to a degree enqueue
In the end,
If there is no point of degree 1, the figure is not DAG
By topological sort can give the DAG node number may be used to determine DAG
Because DAG strict order, there is no connection side from back to front, so you can do dp