leetcode-357 - Count Numbers with Unique Digits

動機

高中排列組合

Problem

Given an integer n, return the count of all numbers with unique digits, x, where 0 <= x < 10n.

Example 1:

Input: n = 2Output: 91Explanation: The answer should be the total numbers in the range of 0 ≤ x < 100, excluding 11,22,33,44,55,66,77,88,99

Example 2:

Input: n = 0Output: 1

Constraints:

• 0 <= n <= 8

Sol

class Solution:
    def countNumbersWithUniqueDigits(self, n: int) -> int:
        if n == 0:
            return 1
        else:
            return sum(9*perm(9,x-1) for x in range(1,n+1))+1