Count Numbers with Unique Digits

 Problem Link: https://leetcode.com/problems/count-numbers-with-unique-digits/

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

 

Example 1:

Input: n = 2
Output: 91
Explanation: 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 = 0
Output: 1

 

Constraints:

• 0 <= n <= 8

Solution (C#):