Distinct Subsequences II LeetCode solution in Python

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class Solution:
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    def distinctSubseqII(self, s: str) -> int:
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        # number of subsequences that startwith each alphabet
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        startwith = [0] * 26
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        for c in s[::-1]:
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            startwith[ord(c) - ord("a")] = sum(startwith) + 1
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        return sum(startwith) % (10**9 + 7)