共通の親ノードを最小化

using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
    class Node
    {
        public Node left;
        public Node right;
        public char data;
        public Node(Node left, Node right, char data)
        {
            this.left = left;
            this.right = right;
            this.data = data;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Node F = new Node(null, null, 'F');
            Node G = new Node(null, null, 'G');
            Node E = new Node(F, G, 'E');
            Node D = new Node(null, null, 'D');
            Node C = new Node(D, E, 'C');
            Node B = new Node(null, null, 'B');
            Node A = new Node(B, C, 'A');

            Node Common = LCA(A, C, G);

            Console.WriteLine(Common.data);
        }

        private static Node LCA(Node root, Node X, Node Y)
        {
            if (root == null) return null;
            if (root == X || root == Y) return root;

            Node left = LCA(root.left, X, Y);
            Node right = LCA(root.right, X, Y);

            if (left == null) return right;
            else if (right == null) return left;
            else return root;
        }
    }
}