Thanks for the feedback. However, I think that the core of the problem is getting this surprise exception and finding out how to deal with it. So the description is not talkative on purpose.

either do a deep copy
or don't do any copy at all, but compute the expected result before you call to the user's function.

Done both

Right. The input data could be forged. I didn't think about that. That wasn't in the spirit of the problem solving, but still possible.
Fixed now, thanks.

I like it. Updated. Slightly changed: i've put the last purpose in the input section.
Marking this as solved.

Alright, now i've caught up with terminology. Edited the description, how is it now?

thanks, fixed

Before editing, there's only one thing I disagree with. I'd say "dictionary of sets of ints" and not "dictionary of sets of successors". I don't see how the sets contain successors: the elements of a set aren't ordered nor contiguous. Only in some example they look like so, but actually they are not.
So, any reason for "successors" to be there?

Yes. I had the doubt about how to represent the adjacency list. I've seen it represented both as list (of sets of successors) and as dictionary.
As a dictionary it is convenient in some cases. In this problem it's the same. Do you think I should change it, and put the list?

About the directed graph, sure, you are right, i'll edit the description. Thanks.

How, exactly? Could you write it in a comment marking it as spoiler?

Dart translation submitted!

Dart translation added.

I had to do research for this. Even knowing the concept, the solution was hard to find.

Yes, kinda. I switched to rank 7, as suggested.

hah! thanks, you have given me a big hint, somehow.
I'll go write that silly solution now...
That's ridiculous. But it also makes sense.