LeetCode: Number of Digit One

LeetCode: 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.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
public class Solution {
    public int countDigitOne(int n) {
        int ones = 0;
        for (long m = 1; m <= n; m *= 10) {
            long a = n / m, b = n % m;
            ones += (a + 8) / 10 * m;

            if (a % 10 == 1) {
                ones += b + 1;
            }
        }
        return ones;
    }
}