public override void AddEdge(object node1, object node2)
{
//create a new index if not already there
if (!AL.ContainsKey(node1))
{
AL.Add(node1, new HashSet <object>());
}
if (!AL.ContainsKey(node2))
{
AL.Add(node2, new HashSet <object>());
}
AL[node1].Add(node2);
AL[node2].Add(node1);
}