class Vertex
{
  // for all i in [0..next.length-1]
  // there is an edge from `this` to `next[i]`   
  Vertex[] next;
  int mark;

  static bool HasCycle(Vertex[] vertices)
  {
    bool Inner(Vertex v)
    {
      if (v.mark == 1) return true;

      v.mark = 1;
      foreach (var n in v.next)
        if (Inner(n)) return true;

      v.mark = 0;
      return false;
    }
    try
    {
      foreach (var v in vertices)
        if (Inner(v)) return true;
    }
    finally
    {
      foreach (var v in vertices)
        v.mark = 0; //clear marks for purity 
    }

    return false;
  }
}