Solution in Java

    int resultIndex = 0;
    Collections.sort(a);

    for (int i = 1; i < a.size(); i++) {
        if (!Objects.equals(a.get(i), a.get(i - 1))) {
            if (i == a.size() - 1){
                resultIndex = i;
                break;
            }
            if (!Objects.equals(a.get(i), a.get(i + 1))) {
                resultIndex = i;
                break;
            }
        }
    }

    return a.get(resultIndex);