Another way to perform topological sorting on a directed acyclic graph G = (V, E) is to

Question:

Another way to perform topological sorting on a directed acyclic graph G = (V, E) is to repeatedly find a vertex of in-degree 0, output it, and remove it and all of its outgoing edges from the graph. Explain how to implement this idea so that it runs in time O (V + E). What happens to this algorithm if G has cycles?