class Solution {
public:
    int countNumbersWithUniqueDigits(int n) {
        if(n == 0){
            return 1;
        }
        if(n == 1){
            return 10;
        }
        int ans = 10, cur = 9;
        for(int i = 0; i < n - 1; i++){
            cur *= 9-i;
            ans += cur;
        }
        return ans;
    }
};
