3871. Count Commas in Range II

You are given an integer n.

Return the total number of commas used when writing all integers from [1, n] (inclusive) in standard number formatting.

In standard formatting:

• A comma is inserted after every three digits from the right.
• Numbers with fewer than 4 digits contain no commas.

class Solution {
public:
    long long countCommas(long long n) {
        long long res = 0;
        for(long long i = 1000, op = 1; i <= n; i = i * 1000, op++) {
            long long cnt = min(i * 1000 - 1, n) - i + 1;
            res += op * cnt;
        }
        return res;
    }
};