using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Node a = new Node { Name = "A" };

        Node a1 = new Node { Name = "A1" };

        Node a11 = new Node { Name = "A11" };

        Node a111 = new Node { Name = "A111" };

        Node a112 = new Node { Name = "A112" };

        Node a113 = new Node { Name = "A113" };

        Node b11 = new Node { Name = "B11" };

        Node b111 = new Node { Name = "B111" };

        Node b112 = new Node { Name = "B112" };

        a.AddChild(a1);

        a1.AddChild(a11);

        a11.AddChild(a111);

        a11.AddChild(a112);

        a11.AddChild(a113);

        a1.AddChild(b11);

        b11.AddChild(b111);

        b11.AddChild(b112);

        Console.WriteLine("Original tree:");

        Console.WriteLine(a.ToString());

        List<Node> selectedNodes = new List<Node> { a111, a112, b111 };

        Console.WriteLine("Selected nodes: " + string.Join(", ", selectedNodes.Select(n => n.Name)));

        Node common = Node.FindClosestCommonAncestor(selectedNodes);

        Console.WriteLine("\r\nClosest common ancestor subtree:");

        Console.WriteLine(common.ToString());

    }

}

​

public class Node

{

    public int Id { get; set; }

    public string Name { get; set; }

    public Node Parent { get; set; }

    public List<Node> Children { get; set; }

    public Node()

    {

        Children = new List<Node>();

    }

    public void AddChild(Node node)

    {

        Children.Add(node);

        node.Parent = this;

    }

    public List<Node> AncestorsAndSelf

    {

        get

        {

            List<Node> list = new List<Node>() { this };

            Node p = Parent;

            while (p != null)

            {

                list.Add(p);

                p = p.Parent;

            }

            return list;

        }

    }

    public int Depth

    {

        get { return AncestorsAndSelf.Count; }

    }

    public static Node FindClosestCommonAncestor(IEnumerable<Node> selectedNodes)

    {

        IEnumerable<Node> commonAncestors = selectedNodes.First().AncestorsAndSelf;

        foreach (Node n in selectedNodes.Skip(1))

        {

            commonAncestors = commonAncestors.Intersect(n.AncestorsAndSelf);

        }

        return commonAncestors.OrderByDescending(n => n.Depth).FirstOrDefault();

    }

    public override string ToString()

    {

        return ToString("");

    }

    private string ToString(string indent)

    {

        string s = indent + Name + "\r\n";

        foreach (Node child in Children)

        {

            s += child.ToString(indent + "    ");

        }

        return s;

    }

}

​