problem:https://leetcode.com/problems/count-numbers-with-unique-digits/
class Solution { public: int countNumbersWithUniqueDigits(int n) { if(n == 0) return 1; int llast = 1; int last = 10; for(int i = 2;i <= n;i++) { int result = last + (last - llast) * (10 - (i - 1)); llast = last; last = result; } return last; } };