public void AddEdge(string from, string to, int weight)
{
Node fromNode = _nodes[from];
if (fromNode == null)
{
throw new Exception();
}
Node toNode = _nodes[to];
if (toNode == null)
{
throw new Exception();
}
fromNode.AddEdge(toNode, weight);
toNode.AddEdge(fromNode, weight);
}