四刷56. Merge Intervals

还是出了点小bug, 没有一开始就反应过来处理intervals == null这些edge cases, 二是没有一开始立刻排序，三是end = Math.max(end,interval.end);这里一开始直接写的end = interval.end;

/**
 * Definition for an interval.
 * public class Interval {
 *     int start;
 *     int end;
 *     Interval() { start = 0; end = 0; }
 *     Interval(int s, int e) { start = s; end = e; }
 * }
 */
class Solution {
    public List merge(List intervals) {
        List res = new ArrayList<>();
        if (intervals == null || intervals.size() == 0){
            return res;
        }
        Collections.sort(intervals, sortByStart);
        int start = intervals.get(0).start;
        int end = intervals.get(0).end;
        for (Interval interval : intervals){
            if (interval.start <= end){
                end = Math.max(end,interval.end);
            } else {
                Interval newInterval = new Interval(start, end);
                res.add(newInterval);
                start = interval.start;
                end = interval.end;
            }
        }
        res.add(new Interval(start, end));
        return res;
    }
    
    private Comparator sortByStart = new Comparator(){
        public int compare(Interval i1, Interval i2){
            return i1.start - i2.start;
        }
    };
}