Topological Sorting

An algorithm for computing a topological ordering of a DAG(Directed
Acyclic Graph) repeatedly finds a node with no incoming edges and
deletes it. This will eventually produce a topological ordering,
provided that the input graph really is a DAG.

But suppose that we're given an arbitrary graph that may or may not
be a DAG. Extend the topological ordering algorithm so that given an
input directed graph G it outputs one of two things: (a) a topological
ordering, thus establishing that G is a DAG; or (b) a cycle in G, and
thus establishing that G is not a DAG. The running time of your
algorithm should be O(m+n) for a directed graph with n nodes and m
edges.