class Program
{
  class Node
  {
      public int Id;
      public Node Parent;
  }

  class TreeNode
  {
      public int Id;
      public List<TreeNode> Nodes;

      public override string ToString()
      {
          return $"(Node {Id} ({string.Join(",", Nodes.Select(node => node.ToString()))}))";
      }
  }

  static void Main(string[] args)
  {
      var node1 = new Node { Id = 1, Parent = null };
      var node2 = new Node { Id = 2, Parent = node1 };
      var node3 = new Node { Id = 3, Parent = node2 };
      var node4 = new Node { Id = 4, Parent = node2 };
      var node5 = new Node { Id = 5, Parent = node2 };
      var node6 = new Node { Id = 6, Parent = node1 };

      var nodes = new[] { node1, node2, node3, node4, node5, node6 };

      var dict = new Dictionary<int, TreeNode>();
      TreeNode root = null;
      foreach (var node in nodes)
      {
          var treeNode = default(TreeNode);
          if (node.Parent == null)
          {

              if (dict.ContainsKey(node.Id))
              {
                  treeNode = dict[node.Id];
              }
              else
              {
                  treeNode = new TreeNode { Id = node.Id, Nodes = new List<TreeNode>() };
                  dict.Add(node.Id, treeNode);
              }
              root = treeNode;
              continue;
          }
          if (dict.ContainsKey(node.Id))
          {
              treeNode = dict[node.Id];
          }
          else
          {
              treeNode = new TreeNode { Id = node.Id, Nodes = new List<TreeNode>() };
              dict.Add(node.Id, treeNode);
          }
          if (dict.ContainsKey(node.Parent.Id))
          {
              var treeParentNode = dict[node.Parent.Id];
              treeParentNode.Nodes.Add(treeNode);
          }
          else
          {
              var treeParent = new TreeNode { Id = node.Parent.Id, Nodes = new List<TreeNode> { treeNode } };
              dict.Add(node.Id, treeParent);
          }
      }

      Console.WriteLine(root.ToString());





      return;
  }
}