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

In case anyone else finds this, but also needs to know the level after they've flattened the tree, this expands on Konamiman's combination of dasblinkenlight and Eric Lippert's solutions:

    public static IEnumerable> FlattenWithLevel(
            this IEnumerable items,
            Func> getChilds)
    {
        var stack = new Stack>();
        foreach (var item in items)
            stack.Push(new Tuple(item, 1));

        while (stack.Count > 0)
        {
            var current = stack.Pop();
            yield return current;
            foreach (var child in getChilds(current.Item1))
                stack.Push(new Tuple(child, current.Item2 + 1));
        }
    }