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.

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)

Solution

来自为知笔记(Wiz)

原文地址：https://www.cnblogs.com/xiejunzhao/p/8445742.html