_233.java

package com.fishercoder.solutions;

/**
 * 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.
 */
public class _233 {

    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;
    }

}