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;    }}