Arithmetic Slices II - Subsequence LeetCode solution in Python

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class Solution:
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    def numberOfArithmeticSlices(self, nums: List[int]) -> int:
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        sz, dp, ans = len(nums), [defaultdict(int) for _ in range(len(nums))], 0
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        for i in range(1, sz):
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            for j in range(i):
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                difference = nums[i] - nums[j]
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                dp[i][difference] += 1
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                if difference in dp[j]:
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                    dp[i][difference] += dp[j][difference]
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                    ans += dp[j][difference]
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        return ans