public IEnumerable <int> PathTo(int v)
{
if (HasPathTo(v))
{
var path = new NodeStack <int>();
for (int i = v; i != _s; i = _pathTo[i])
{
path.Push(i);
}
path.Push(_s);
while (!path.IsEmpty())
{
yield return(path.Pop());
}
}
;
}