Given k integer lists all of equal length, return the number of tuples (index[i] for 0 <= i < k) such that sum(arrays[i][index[i]] for 0 <= i < k) == 0.

k_sum.py

from collections import Counter
from itertools import product
class Solution:
    def fourSumCount(self, *lists):
        k = len(lists)
        sum_to_count = Counter(sum(nums) for nums in product(*lists[:k // 2]))
        return sum(sum_to_count[-sum(nums)] for nums in product(*lists[k // 2:]))