233. Number of Digit One

class Solution {
    public int countDigitOne(int n) {
        if (n == 0) return 0;

        long base = 1;
        int sum = 0;

        while (base <= n) {
            int cur = (int) (n / base % 10);
            int left = (int) (n / base / 10);
            int right = (int) (n % base);
            
            if (cur > 1) {
                sum += (left + 1) * base;
            } else if (cur == 1) {
                sum += (left) * base;              
                sum += (right + 1);
            } else {
                sum += left * base;
            }

            base *= 10;
        }

        return sum;
    }
}