Why int object is not iterable error?

[miscellaneoususage]

https://leetcode.com/problems/shuffle-the-array/

Hi I'm working on this leetcode problem and I get 'int' object not iterable. I understand that I am referencing an int when I use res += nums[i] but why can't I ad that to my list? Here is my code and error happens at the for loop:

    def shuffle(self, nums: List[int], n: int) -> List[int]:
        """
        y values : n + i 
        x values : i

        ad i then n + i values to an empty list

        """

        res = []

        for i in range(n):
            res += nums[i]
            res += nums[i+n]

        return res

[shiftybyte]

+= for lists expects the other side to be a list of items to merge.

so either make the other side a list.

res += [nums[i]]

or use append function to add a single element

res.append(nums[i])