在二郎/长生不老药有向图中寻找树根

我有以下超简单的有向图树(代码是Elixir):

digraph = :digraph.new()
coords = [{0.0, 0.0}, {1.0, 0.0}, {1.0, 1.0}]
[v0, v1, v2] = (for c <- coords, do:   :digraph.add_vertex(digraph, c))
:digraph.add_edge(digraph, v0, v1)
:digraph.add_edge(digraph, v1, v2)

iex(82)> :digraph_utils.is_tree(digraph)
true

但是如何有效地找到树的顶点根呢?目前我正在做:

iex(83)> Enum.filter(:digraph.vertices(digraph), fn x -> :digraph.in_degree(digraph, x) == 0 end)
[{0.0, 0.0}]

这是找到根顶点最有效的方法吗?也就是说,找到没有边的顶点?