Binary Subarrays With Sum LeetCode solution in Python

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# Runtime: 680 ms (Top 7.52%) | Memory: 17.5 MB (Top 60.81%)
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class Solution:
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    def numSubarraysWithSum(self, nums: List[int], goal: int) -> int:
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        my_dict = {0: 1}
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        accum, res = 0, 0
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        for n in nums:
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            accum += n
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            diff = accum - goal
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            if diff in my_dict:
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                res += my_dict[diff]
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            if accum in my_dict:
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                my_dict[accum] += 1
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            else:
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                my_dict[accum] = 1
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        return res