357. Count Numbers with Unique Digits 用唯一的数字计算数字

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

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])

Credits:
Special thanks to @memoryless for adding this problem and creating all test cases.

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class Solution:     def countNumbersWithUniqueDigits(self, n):         """         :type n: int         :rtype: int         """         used = [False for x in range(10)]           def gen(used, n, d):             if n == d:                 return 1             total = 1             startIndex = 1 if d == 0 else 0             for i in range(startIndex, 10):                 if not used[i]:                     used[i] = True                     total += gen(used, n, d + 1)                     used[i] = False             return total           return gen(used, n, 0)     s = Solution() a = s.countNumbersWithUniqueDigits(7) print(a)

来自为知笔记(Wiz)