357. Count Numbers with Unique Digits

    /*
     * 357. Count Numbers with Unique Digits
     * 2016-7-13 by Mingyang
     * DP
     */
    

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;
    }