How would you write the pseudocode for this problem?

The algorithm described in Section $3\mathrm{.}6$ for computing a topological order$-$

ing of a DAG 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$,$ 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.