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

You can flatten a tree like this:

IEnumerable Flatten(IEnumerable e) =>
    e.SelectMany(c => Flatten(c.Elements)).Concat(new[] { e });

You can then filter by group using Where(...).

To earn some "points for style", convert Flatten to an extension function in a static class.

public static IEnumerable Flatten(this IEnumerable e) =>
    e.SelectMany(c => c.Elements.Flatten()).Concat(e);

To earn more points for "even better style", convert Flatten to a generic extension method that takes a tree and a function that produces descendants from a node:

public static IEnumerable Flatten(
    this IEnumerable e
,   Func> f
) => e.SelectMany(c => f(c).Flatten(f)).Concat(e);

Call this function like this:

IEnumerable tree = ....
var res = tree.Flatten(node => node.Elements);

If you would prefer flattening in pre-order rather than in post-order, switch around the sides of the Concat(...).