1067. Digit Count in Range

Given a single-digit integer d and two integers low and high, return the number of times that d occurs as a digit in all integers in the inclusive range [low, right].

class Solution {
    long helper(int num, int d) {
        long right = num, left = 0;
        long sum = 0, p = 1;
        while(right) {
            int n = right % 10;
            right /= 10;
            
            sum += right * p;
            if(d) {
                if(n == d) sum += left + 1;
                else if(n > d) sum += p;
                left = left + p * n;
            } else {
                left = left + p * n;
                if(right and p > 1) {
                    sum -= p;
                    sum += min(left + 1, p);
                }
            }
            p *= 10;
        }
        return sum;
    }
public:
    int digitsCount(int d, int low, int high) {
        return helper(high, d) - helper(low - 1, d);
    }
};