How to use void for writing your solutions ?

Question

Hello I came across a question in an interview which states as follows and I have to write another version of this using the instructions. I got confused as to how:

void Foo(TreeNode root)
  {
  Stack nodes = new Stack();
  nodes.Push(root);
 
  while (nodes.Count > 0)
  {
  TreeNode node = (TreeNode) nodes.Pop();
  Console.WriteLine(node.Text);
  for (int i = node.Nodes.Count - 1; i >= 0; i--)
  nodes.Push(node.Nodes[i]);
  }
  }






This is the diagram do not use stack class. Write a shorter and better solution to achieve this. start with
void Foo(TreeNode root)
{
 
}
---------------------------------------------------------------------------------

I managed to do it like below but do not knowhow to write using void Foo(TreeNode root)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PleasantSoln
{
  public class Program
  {

  List<string> Treenode = new List<string>();
  public static void Main()
  {
 
  List<string> Treenode = new List<string>();

  Treenode.Add("Root");
  Treenode.Add("Trunk");
  Treenode.Add("Branches");
  Treenode.Add("Leaves");


  Console.WriteLine();
  foreach (string node in Treenode)
  {
  Console.WriteLine(node);
  }

  Treenode.Reverse();

  Console.WriteLine();
  foreach (string node in Treenode)
  {
  Console.WriteLine(node);
  }

  Treenode.Reverse(1, 3);

  Console.WriteLine();
  foreach (string node in Treenode)
  {
  Console.WriteLine(node);
  Console.ReadKey();
  }
  }
  }
}