leetcode 357. Count Numbers with Unique Digits

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Given a non-negative integer n, count all numbers with unique digits, x, where 0 ≤ x < 10^n.

Example:
Given n = 2, return 91. (The answer should be the total numbers in the range of 0 ≤ x < 100, excluding [11,22,33,44,55,66,77,88,99])

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class Solution(object):
    def countNumbersWithUniqueDigits(self, n):
        """
        :type n: int
        :rtype: int
        """
        res = 1
        for i in range(n):
                cur = 9
                for j in range(i): cur *= 9 - j
                res += cur
        return res