Задача: Одинокие узлы двоичного дерева

GetLonelyNodes.cs

using System;
using System.Collections.Generic;
					
public class Program
{
	public class TreeNode 
	{
		public int val;
		public TreeNode left;
		public TreeNode right;
		public TreeNode(int val, TreeNode left = null, TreeNode right = null) 
		{
			this.val = val;
			this.left = left;
			this.right = right;
		}
	}
	
	public static List<int> LonelyNodes;
    
	public static IList<int> GetLonelyNodes(TreeNode root) 
	{
		LonelyNodes = new List<int>();

		if (root == null)
		    return LonelyNodes;

		Helper(root);

		return LonelyNodes;
	}

	public static void Helper(TreeNode node)
	{
		if (node == null)
		    return;

		if (node.left == null && node.right != null)
		    LonelyNodes.Add(node.right.val);

		if (node.left != null && node.right == null)
		    LonelyNodes.Add(node.left.val);

		Helper(node.left);
		Helper(node.right);
	}
	
	public static void Main()
	{
		Console.WriteLine("UniLecs");
		
		// test
		var root = new TreeNode(2);
		root.left = new TreeNode(7);
		root.right = new TreeNode(5);
		root.left.left = new TreeNode(2);
		root.left.right = new TreeNode(6);
		root.left.right.left = new TreeNode(5);
		root.left.right.right = new TreeNode(11);
		
		root.right.right = new TreeNode(9);
		root.right.right.left = new TreeNode(4);
		
		Console.WriteLine(string.Join(", ", GetLonelyNodes(root))); // [9, 4]
	}
}