How to remove excess brackets in a 3d list to make a 2d list?

I am doing a leetcode problem named 3Sum: Here is my code:

class Solution:
    def threeSum(self, nums: List[int]) -> List[List[int]]:
        def twoSum(nums, target):
            d = {
                target - nums[0]: 0 
            }

            for i in range(1, len(nums)):
                if nums[i] in d:
                    return [nums[i], target - nums[i]]
                else:
                    d[target-nums[i]] = i
        arr = []

        for i in range(len(nums)):
            s = twoSum(nums[i+1:] + nums[:i], -nums[i])
            if s == None:
                continue
            if s not in arr:
                arr.append(s)
                arr[len(arr) - 1].append(nums[i])
        
        n = list(map(sorted, arr))
        print(n)
        res = []
        [res.append(x) for x in n if x not in res]
        return res

It isn't the greatest of solutions, but I ran into a problem. I noticed that I couldn't just implement my 2Sum solution into it because, to put it simply, while the 2Sum problem guarantees that there is only one solution, there could be many solutions for 3Sum. Therefore, I thought instead of just returning one list, how about a return a 2d list, so that the outer list can hold the many solutions.

However, that raised another problem, the one I hope the answer to this problem will solve. I'm then appending that 2Sum into the arr list. In doing so, arr becomes a 3d list, which I would like to be a 2d list.

I am wondering how to make a list like this: arr = [[[3, 2, 9, 4], [1, 9, 5]], [3, 2, 2]] into a list like this: arr = [[3, 2, 9, 4], [1, 9 ,5], [3, 2, 2]], so that it meets the problem requirements. In other words, how to remove only the brackets that make the list a 3d list so that it becomes a 2d list. The data type of arr is a list.

Notice1: I understand that in the problem, the subarrays of arr would only have 3 elements each, but I would like to generalize it so that the answer to this question isn't specific to only this leetcode problem.

Notice2: I want [[3, 2, 9, 4], [1, 9 ,5], [3, 2, 2]] instead of [[3, 2, 9, 4, 1, 9 ,5], [3, 2, 2]], because the second list also removes some brackets of the 2d lists. Hope that makes sense.

Another example is arr being equal to [[[5]], [3, 2, 3, 5, 10]]. If it were that case, I would like to make it [[5], [3, 2, 3, 5, 10]], so that it is only a 2d array.

One last example is [[[2, 6, 3, 1, 9], [3, 1, 99, 4]], [[4, 15], [4, 7, 2, 1]]]. That would turn into [[2, 6, 3, 1, 9], [3, 1, 99, 4], [4, 15], [4, 7, 2, 1]].

Thank you!

Loop over the list. If an element is a 2d list, extend the result to flatten it, otherwise append the element onto the result.

data = [[[3, 2, 9], [1, 9, 5]], [3, 2, 2]]
result = []
for i in data:
    if isinstance(i[0], list):
        result.extend(i)
    else:
        result.append(i)

print(result)