In a directed graph, the number of edges coming out of v is the out$-$degree and the

number of edges coming into v is the in$-$degree. Explain why a DAG must have at least

one vertex that has in$-$degree $0.$ Explain why a DAG must have at least one vertex

that has out$-$degree $0.$ Use this fact to describe a recursive algorithm that outputs a

topological ordering for a DAG G$.$ Do not use DFS in your solution! Explain the runtime

of your algorithm