Ah, that is even more vicious, warning people in advance so thay can't make excuses, how wicked.

Sorry...

Now I have an other idea with something like n.ln(n) + 10n complexity. I haven't finished it yet but when I try to run it with a return None just after the n.ln(n) portion, it is still running out of time, so I suppose that it will be worse when I'll be finished. Is it normal, and I should search for a better complexity or there is a problem with my code that is actually more complex than n.ln(n) ?

And thank you for your guiding.

edited: it seems there is a solution with O(n) complexity