Given a collection of intervals, merge all overlapping intervals.

For example,
Given[1,3],[2,6],[8,10],[15,18],
return[1,6],[8,10],[15,18].

/**
 * 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<Interval> merge(List<Interval> intervals) {
        List<Interval> ans = new ArrayList<>();
        if (intervals == null || intervals.size() == 0) return ans;

        Collections.sort(intervals, (a, b) -> (a.start - b.start));
        Interval last = intervals.get(0);
        for (int i = 1; i < intervals.size(); i++){
            Interval cur = intervals.get(i);

            if (cur.start > last.end){
                ans.add(last);
                last = cur;
            }
            else{
                last.end = Math.max(last.end, cur.end);
            }
        }
        ans.add(last);

        return ans;
    }
}