Number of Digit One

Problem: Number of Digit One

Code in Python:

class Solution(object):
    def countDigitOne(self, n):
        """
        :type n: int
        :rtype: int
        """
        if n < 0: return 0

        count, num, base, highBase = 0, n, 1, 10
        while num:
            num, mod = divmod(num, 10)

            if mod == 0: count += (n / highBase) * base
            elif mod == 1: count += (n / highBase) * base + (n % base + 1)
            else: count += (n / highBase + 1) * base

            base *= 10
            highBase *= 10

        return count