public static void Test(){
Node root = new Node(1);
root.Left = new Node(2);
root.Right = new Node(3);
root.Left.Left = new Node(4);
root.Left.Right = new Node(5);
root.Right.Left = new Node(6);
root.Right.Right = new Node(7);
Siblings sb = new Siblings();
sb.AttachSiblings(root);
Console.Write(root.Right.Right.Prev.Data+" == 6");
Console.Write("\n"+root.Right.Left.Prev.Data+" == 5");
Console.Write("\n"+(root.Left.Left.Prev==null?"null":"exists")+" == null");
}