// check that algorithm computes either the topological order or finds a directed cycle
private void dfs(EdgeWeightedDigraph G, int v)
{
// Console.Out.WriteLine(cost++);
onStack[v] = true;
marked[v] = true;
foreach(DirectedEdge e in G.adj(v))
{
DirectedEdge resultEdge = e;
int w = resultEdge.to();
// short circuit if directed cycle found
if (_cycle != null) return;
//found new vertex, so recur
else if (!marked[w]) {
edgeTo[w] = resultEdge;
dfs(G, w);
}
// trace back directed cycle
else if (onStack[w]) {
_cycle = new Stack<DirectedEdge>();
while (e.from() != w) {
_cycle.Push(resultEdge);
resultEdge = edgeTo[e.from()];
}
_cycle.Push(resultEdge);
}
}
onStack[v] = false;
}
