357.md

头尾的特殊条件处理：n = 0时的1和起始第二位的9

int countNumbersWithUniqueDigits(int n) {
    int ans = 10, tmp = 9;
    for(int i = 1; i < n && tmp; ++i)
        ans += (tmp *= (10-i));
    return (n == 0)?1:ans;
}