Number of Digit One

Given an integer n, count the total number of digit 1 appearing in all non-negative integers less than or equal to n.

For example:
Given n = 13,
Return 6, because digit 1 occurred in the following numbers: 1, 10, 11, 12, 13.

思路: http://blog.csdn.net/xudli/article/details/46798619

public class Solution {
    public int countDigitOne(int n) {
         int res = 0;
         for (long m = 1; m <= n; m*= 10) {
             long a = n / m;
             long b = n % m;
             res += (a + 8) / 10 * m;
             if (a % 10 == 1) {
                 res += b + 1;
             }
         }
         return res;
    }
}