Numbers With Repeated Digits LeetCode solution in Python

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class Solution:
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    def numDupDigitsAtMostN(self, n: int) -> int:
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        nums = [int(i) for i in str(n + 1)]  # digits in n+1
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        d = len(nums)  # number of digits in n+1
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        res = 0  # number of no duplicates
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        # count no duplicates for numbers with <d digits
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        for i in range(1, d):
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            res += 9 * math.perm(9, i - 1)
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        # count no duplicates for numbers with d digits and smaller than n
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        for i, x in enumerate(nums):
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            if i == 0:
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                digit_range = range(1, x)  # first digit can not be 0
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            else:
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                digit_range = range(x)
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            for y in digit_range:
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                if y not in nums[:i]:
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                    res += math.perm(9 - i, d - 1 - i)
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            if x in nums[:i]:
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                break
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        return n - res