How to flatten tree via LINQ?

Question

So I have simple tree:

class MyNode
{
 public MyNode Parent;
 public IEnumerable Elements;
 int group = 1;
}

I have a I

Answer 1

Just for completeness, here is the combination of the answers from dasblinkenlight and Eric Lippert. Unit tested and everything. :-)

 public static IEnumerable Flatten(
        this IEnumerable items,
        Func> getChildren)
 {
     var stack = new Stack();
     foreach(var item in items)
         stack.Push(item);

     while(stack.Count > 0)
     {
         var current = stack.Pop();
         yield return current;

         var children = getChildren(current);
         if (children == null) continue;

         foreach (var child in children) 
            stack.Push(child);
     }
 }