Question

I have the following directed graph implementation

 Nodes nod[]
 List<Arcs> arc[]

So the node on the n’th position has all his arcs of the list in position n. Of course, the nodes are organized accordingly, so that I can use Binary Search.
Based on this implementation. I wish to create a DFS algorithm. I know very well the pseudo code, adapting to java shouldn’t be a problem.

But my question is the following. In DFS we need to start searching from the “top” node. And thinking about it, i dont have this “top” node. Moreover, I have no idea how to get it.
So I ask, how do i get this top node, considering my implementation?

Thanks for the help.

Answer 1

Expanding on my comment:

Assuming that the arcs are directed (i.e., from the parent node to the child only) you can search all of the nodes for the one with no incoming arcs:

// parent_count is an integer array of the same size as nod[]

for i = 1..n
    for each arc in arc[i] (arc going from i to j)
        increment parent_count[j]
    end
end

for k = 1..n
    if parent_count[k] == 0
        return k
end