using System;

public class Program

{

    public static void Main()

    {

        //This block is creating required Tree structure as per the image

        Node Core = new Node() { Value = "Core", ParentNode=null};

        Node First = new Node(){ Value = "First", ParentNode=Core};

        Node A1 = new Node(){ Value = "A1", ParentNode= First}; 

        Node A2 = new Node(){ Value = "A2", ParentNode= First}; 

        Node Second = new Node(){ Value = "Second", ParentNode=Core};

        Node B1 = new Node(){ Value = "B1", ParentNode= Second};    

        Node B2 = new Node(){ Value = "B2", ParentNode= Second};    

        Node Third = new Node(){ Value = "Third", ParentNode=Core};

        Node C1 = new Node(){ Value = "C1", ParentNode= Third}; 

        Node CC3 = new Node(){ Value = "CC3", ParentNode= C1};  

        Node CC4 = new Node(){ Value = "CC4", ParentNode= C1};

        Node CC5 = new Node(){ Value = "CC5", ParentNode= C1};

        Node C2 = new Node(){ Value = "C2", ParentNode= Third};

        Node C3 = new Node(){ Value = "C3", ParentNode= Third}; 

        Node CC1 = new Node(){ Value = "CC1", ParentNode= C3};  

        Node CC2 = new Node(){ Value = "CC2", ParentNode= C3};

        Node C4 = new Node(){ Value = "C4", ParentNode= Third};

        Node Forth = new Node(){ Value = "Forth", ParentNode=Core};

        Node D1 = new Node(){ Value = "D1", ParentNode= Forth}; 

        Node D2 = new Node(){ Value = "D2", ParentNode= Forth};

        Node D3 = new Node(){ Value = "D3", ParentNode= Forth};

        Node Fifth = new Node(){ Value = "Fifth", ParentNode=Core};

        Node E1 = new Node(){ Value = "E1", ParentNode= Fifth}; 

        Node E2 = new Node(){ Value = "E2", ParentNode= Fifth};

        Node EE1 = new Node(){ Value = "EE1", ParentNode= E1};  

        Node EEE1 = new Node(){ Value = "EEE1", ParentNode= EE1};   

​

        Node EE2 = new Node(){ Value = "EE2", ParentNode= E1};

        Node EE3 = new Node(){ Value = "EE3", ParentNode= E1};

        Console.WriteLine("Tree Generated");

        Node Result = FindCommonParent(A1, A2);     

        Console.WriteLine("Common Parent : " + Result.Value);

    }

    public static Node FindCommonParent(Node input1, Node input2)

    {           

        int level1 = GetLevel(input1);

        int level2 = GetLevel(input1);

        Node Result  = new Node(); 

        Console.WriteLine("Level1 : " + level1.ToString());

        Console.WriteLine("Level2 : " + level2.ToString());

            if(level1>level2) //It means that Parent1 is in below level compared to Parent2. So Parent2 will be still

            {

                for(int i=level1; i>=level2; i--)

                {

                    Console.WriteLine("Parent1 : " + input1.Value);

                    Console.WriteLine("Parent2 : " + input1.Value);

                    input1  = GetParentNode(input1);    

                    if(input1.Value == input2.Value)

                    {

                        Result.Value = input1.Value;

                        return Result;

                    }                   

                }               

            }

            else

            {

                for(int i=level2; i>=level1; i--)

                {

                    input2  = GetParentNode(input2);    

                    if(input1.Value == input2.Value)

                    {

                        Result.Value = input1.Value;

                        return Result;

                    }                   

                }   

            }

        return Result;

    }

    public static int GetLevel(Node input)

    {

        int level=0;

        do

        {

            level++;

            input=input.ParentNode;

        }while(input!=null);

        return level;

    }

    public static Node GetParentNode(Node input)

    {

        Node Parent = new Node();

        if(input.ParentNode != null)

        {

            Parent.Value = input.ParentNode.Value;

            Parent.ParentNode = input.ParentNode.ParentNode;

        }

        return Parent;

    }

}

​

public class Node

{

    public string Value {get; set;}

    public Node ParentNode { get; set;}

}

​

​

​