题目：

题解：

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;}
};