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

Below is Ivan Stoev's code with the additonal feature of telling the index of every object in the path. E.g. search for "Item_120":

Item_0--Item_00
        Item_01

Item_1--Item_10
        Item_11
        Item_12--Item_120

would return the item and an int array [1,2,0]. Obviously, nesting level is also available, as length of the array.

public static IEnumerable<(T, int[])> Expand(this IEnumerable source, Func> getChildren) {
    var stack = new Stack>();
    var e = source.GetEnumerator();
    List indexes = new List() { -1 };
    try {
        while (true) {
            while (e.MoveNext()) {
                var item = e.Current;
                indexes[stack.Count]++;
                yield return (item, indexes.Take(stack.Count + 1).ToArray());
                var elements = getChildren(item);
                if (elements == null) continue;
                stack.Push(e);
                e = elements.GetEnumerator();
                if (indexes.Count == stack.Count)
                    indexes.Add(-1);
                }
            if (stack.Count == 0) break;
            e.Dispose();
            indexes[stack.Count] = -1;
            e = stack.Pop();
        }
    } finally {
        e.Dispose();
        while (stack.Count != 0) stack.Pop().Dispose();
    }
}