leetcode 357. Count Numbers with Unique Digits

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class Solution 
{
public:
    int countNumbersWithUniqueDigits(int n) 
    {
        int res = 1;
        int permNumber = 9;
        for(int i = 1;i<=n && i<= 10;i++)
        {
            res += permNumber;
            permNumber *= (10-i);
        }
        return res;
    }
};