This helped a lot. Thanks. Haha

TypeError: 'int' object is not iterable ???

I brute forced it

exactly like yours... (but what is even N, in there??)

meh, O(n^2)

I broke my head trying to wrap it around this generator. To understand it easier consider writing it like this:

[number for subarray in array for number in subarray]

Still not easy though :|

I wated to do this, but I didn't know how to do a double for in a single line, cool

The brackets [] are not needed inside sorted. sorted can accept a generator)

The problems here is to avoid loops by finding a formula (or more) that can get the job done quickly. I managed to find a crappy solution but still with one loop (hovewer I saw some solutions with zero loops at all). By typing the inputs of the kata on oeis.org it will return different algorythm with all kind of details. By looking at all this informations and (tinkering a lot) you may find some insight to solves this kata. (Also, leaving the keyborg and using some pencil and paper help me a lot for the tinkering part.)

could you please tell what pattern, I have to search

I can't even understand what's going on here... Anyone care to elaborate?

check the description in the question there is just one situation where the response should be empty string.
Hint: you don't have to check for empty categories in go.

I run python 3.6.1 and I didn't noticed I could change the python's version on codewars. So I changed it to 3.6 and now my code works.
Thanks @rowcased for the input.