leetcode357

public class Solution {
    public int CountNumbersWithUniqueDigits(int n) {
        if (n == 0)
            {
                return 1;
            }
            int res = 10;
            int uniqueDigits = 9;
            int availableNumber = 9;
            while (n-- > 1 && availableNumber > 0)
            {
                uniqueDigits = uniqueDigits * availableNumber;
                res += uniqueDigits;
                availableNumber--;
            }
            return res;
    }
}

https://leetcode.com/problems/count-numbers-with-unique-digits/#/description