leetcode MergeIntervals

import utils.Interval;

import java.util.*;

public class MergeIntervals {
    /**
     * Given a collection of intervals, merge all overlapping intervals.
     *
     * Example 1:
     *
     * Input: [[1,3],[2,6],[8,10],[15,18]]
     * Output: [[1,6],[8,10],[15,18]]
     * Explanation: Since intervals [1,3] and [2,6] overlaps, merge them into [1,6].
     * Example 2:
     *
     * Input: [[1,4],[4,5]]
     * Output: [[1,5]]
     * Explanation: Intervals [1,4] and [4,5] are considered overlapping.
     */
    public List<Interval> merge(List<Interval> intervals) {
        List<Interval> ans = new ArrayList<>();
        Collections.sort(intervals, new Comparator<Interval>() {
            @Override
            public int compare(Interval o1, Interval o2) {
                return o1.start >= o2.start ? -1: 1;
            }
        });
        boolean startIsNull = true;
        int start = 0;
        int end = 0;
        for (int i=0; i<intervals.size(); i++) {
            if (startIsNull) {
                start = intervals.get(i).start;
                end = intervals.get(i).end;
                startIsNull = false;
            }
            if (intervals.get(i).start <= end) {
                end = Math.max(end, intervals.get(i).end);
            }
            else {
                ans.add(new Interval(start, end));
                startIsNull = true;
            }
        }
        if (startIsNull) {
            ans.add(new Interval(start, end));
        }
        return ans;
    }
}