The alternative solution should be corrected like the first solution.

It should cast the arr.size() to int to mod with negative value.

Without casting, mod result will be affected by overflow because of signed status difference between int & size_t

        int nextIndex = (currentIndex + arr[currentIndex]) % (int)arr.size();
        if (nextIndex < 0) {
            nextIndex += arr.size(); // wrap around for negative numbers
        }