hmmm, that gives me an idea, what if in my current code, instead of going from arrival time to departure time trying to find a free slot, I do it backwards like this....

Too bad I can't access the HackerRank anymore, but this seems to work on my online compiler with my test case. Thoughts?

public static int countMeetings(List<Integer> arrival, List<Integer> departure) {
    if (arrival.isEmpty() || departure.isEmpty()){
        return 0;
    }

    int maxMeetings = 0;
    Set<Integer> booked = new HashSet<>();

    // books time slots according to first come first serve
    // need to optimize time slots for maximum meetings
    for (int i = 0; i < arrival.size(); i++){
        for (int j = departure.get(i); j >= arrival.get(i); j--){
            if (!booked.contains(j)){
                maxMeetings++;
                booked.add(j);
                break;
            }
        }
    }

    return maxMeetings;
}