4Sum II LeetCode solution in Python

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# Runtime: 1360 ms (Top 29.95%) | Memory: 14.2 MB (Top 70.39%)
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class Solution:
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    def fourSumCount(
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        self, nums1: List[int], nums2: List[int], nums3: List[int], nums4: List[int]
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    ) -> int:
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        n1n2 = defaultdict(int)
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        for n1 in nums1:
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            for n2 in nums2:
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                n1n2[n1 + n2] += 1
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        n3n4 = defaultdict(int)
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        for n3 in nums3:
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            for n4 in nums4:
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                n3n4[n3 + n4] += 1
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        ans = 0
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        for s in n1n2:
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            ans += n1n2[s] * n3n4[-s]
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        return ans