3790. Smallest All-Ones Multiple

You are given a positive integer k.

Find the smallest integer n divisible by k that consists of only the digit 1 in its decimal representation (e.g., 1, 11, 111, …).

Return an integer denoting the number of digits in the decimal representation of n. If no such n exists, return -1.

class Solution {
public:
    int minAllOneMultiple(int k) {
        unordered_set<int> seen;
        int rem = 1, res = 2, val = 10 % k;
        while(1) {
            rem = (rem + val) % k;
            if(rem == 0) return res;
            if(seen.count(rem)) return -1;
            seen.insert(rem);
            val = (val * 10) % k;
            res++;
        }
        return -1;
    }
};