number-of-digit-one.cpp

// Time:  O(logn) = O(1)
// Space: O(1)

class Solution {
public:
    int countDigitOne(int n) {
        const int k = 1;
        int cnt = 0, multiplier = 1, left_part = n;

        while (left_part > 0) {
            // split number into: left_part, curr, right_part
            int curr = left_part % 10;
            int right_part = n % multiplier;

            // count of (c000 ~ oooc000) = (ooo + (k < curr)? 1 : 0) * 1000
            cnt += (left_part / 10 + (k < curr)) * multiplier;

            // if k == 0, oooc000 = (ooo - 1) * 1000
            if (k == 0 && multiplier > 1) {
                cnt -= multiplier;
            }

            // count of (oook000 ~ oookxxx): count += xxx + 1
            if (curr == k) {
                cnt += right_part + 1;
            }

            left_part /= 10;
            multiplier *= 10;
        }

        return cnt;
    }
};