/// <summary>
/// return a reverse version of digraph
/// </summary>
/// <returns></returns>
public Digraph Reverse()
{
Digraph R = new Digraph(v);
for (int i = 0; i < v; i++)
{
foreach (int w in Adj(i))
{
R.AddEdge(w, i);
}
}
return(R);
}
