Number of Digit One

Description：

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.

Code：

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