src/main/java/com/fishercoder/solutions/_233.java

package com.fishercoder.solutions;

public class _233 {
    public static class Solution1 {
        public int countDigitOne(int n) {
            int count = 0;
            for (long k = 1; k <= n; k *= 10) {
                long r = n / k;
                long m = n % k;
                // sum up the count of ones on every place k
                count += (r + 8) / 10 * k + (r % 10 == 1 ? m + 1 : 0);
            }
            return count;
        }
    }
}