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.
Hint:
Beware of overflow.
Solution:
1. public class Solution {
2. public int countDigitOne(int n) {
3. int count = 0;
4. 5. for (long k = 1; k <= n; k *= 10) {
6. long r = n / k, m = n % k;
7. count += (r + 8) / 10 * k + (r % 10 == 1 ? m + 1 : 0);
8. }
9. 10. return count;
11. }
12. }