LeetCode - Merge Intervals

public List<Interval> merge(List<Interval> intervals) {
    Map<Integer, Integer> map = new TreeMap<>();
    for(Interval interval : intervals){
        if(map.containsKey(interval.start))
            map.put(interval.start, Math.max(interval.end, map.get(interval.start)));
        else
            map.put(interval.start, interval.end);
    }
    int preStart = -1, preEnd = -1;
    List<Interval> result = new LinkedList<>();
    for(Map.Entry<Integer, Integer> entry : map.entrySet()){
        if(preStart == -1){
            preStart = entry.getKey();
            preEnd = entry.getValue();
        }else{
            if(entry.getKey() <= preEnd){
                preStart = Math.min(preStart, entry.getKey());
                preEnd = Math.max(preEnd, entry.getValue());
            }else{
                result.add(new Interval(preStart, preEnd));
                preStart = entry.getKey();
                preEnd = entry.getValue();
            }
        }
    }
    if(preStart != -1){
        result.add(new Interval(preStart, preEnd));
    }
    return result;
}