3870. Count Commas in Range

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:
    int countCommas(int n) {
        int res = 0;
        for(int i = 1000, op = 0, nxt = 1000; i <= n; i++) {
            if(i == nxt) {
                nxt = nxt * 1000;
                op++;
            }
            res += op;
        }
        return res;
    }
};