editorial

ConcatenateNumber.java

    // - number theory: r = (r * p10 + number) % k
    // - 32bit int overflow because of multiplication
    // - cycle dectection: cyclen length < k, only need to iterate k times
    public int getSmallest(int number, int k) {
        long r = number % k, n = number, p10 = 1;
        while (n > 0) {
            n /= 10;
            p10 *= 10;
        }
        for (int i = 1; i <= k; ++i) {
            if (r == 0) return i;
            r = (r * p10 + number) % k;
        }
        return -1;
    }