using System;
    using System.Linq;
    using System.Collections.Generic;
     
    public static class MyExtensions
    {
    	public static IEnumerable<T> Traverse<T>(
    	    this IEnumerable<T> source,
    	    Func<T, IEnumerable<T>> fnRecurse)
    	{
    		if (source != null)
    		{
    			Stack<IEnumerator<T>> enumerators = new Stack<IEnumerator<T>>();
    			try
    			{
    				enumerators.Push(source.GetEnumerator());
    				while (enumerators.Count > 0)
    				{
    					var top = enumerators.Peek();
    					while (top.MoveNext())
    					{
    						yield return top.Current;
     
    						var children = fnRecurse(top.Current);
    						if (children != null)
    						{
    							top = children.GetEnumerator();
    							enumerators.Push(top);
    						}
    					}
     
    					enumerators.Pop().Dispose();
    				}
    			}
    			finally
    			{
    				while (enumerators.Count > 0)
    					enumerators.Pop().Dispose();
    			}
    		}
    	}
    }
     
    public class Node
    {
    	public int Id;
    	public List<Node> Children;
    }
     
    public class Test
    {
    	public static void Main()
    	{
    		List<Node> nodes = new List<Node>()
    		{
    			new Node()
    			{
    				Id = 1,
    				Children = new List<Node>()
    				{
    					new Node()
    					{
    						Id = 2,
    						Children = new List<Node>()
    						{
    							new Node()
    							{
    								Id = 4
    							},
    							new Node()
    							{
    								Id = 5
    							}
    						}
    					},
    					new Node()
    					{
    						Id = 3
    					}
    				}
    			}
    		};
     
    		Console.WriteLine(String.Join(", ", nodes.Traverse(n => n.Children).Select(n => n.Id)));
    	}
    }