357. Count Numbers with Unique Digits

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

代码：

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