Hello I want to enumerate all vertices with zero input edge count in a graph, I am using DFS right now:

struct vertex_visitor : public boost::default_dfs_visitor
    {
    std::vector<vertex_t> Output;

    void discover_vertex(vertex_t v, graph_t const& g)
        {
        graph_t::in_edge_iterator edgeIt, edgeEnd;
        tie(edgeIt, edgeEnd) = boost::in_edges(v, g);
        if (edgeIt==edgeEnd)
            Output.push_back(v);
        }
    };

vertex_visitor vis;
boost::depth_first_search(g, boost::visitor(v));