剑指offer——45整数中1出现的次数

 1 class Solution {
 2 public:
 3     int NumberOf1Between1AndN_Solution(int n)
 4     {
 5         if (n <= 0)return 0;
 6         int  left = 0, right = 0, a = 1, now = 1, ans = 0;
 7         while (n / a) {
 8             left = n / (a * 10), now = n / a % 10, right = n % a;
 9             if (now == 0) ans += left * a;
10             else if (now == 1) ans += left * a + right + 1;
11             else ans += (left + 1) * a; 
12             a = a * 10;
13         }
14         return ans;
15     }
16 };