Merge Intervals

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].

Solution:

/**
 * 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; }
 * }
 */
public class Solution {
    public List<Interval> merge(List<Interval> intervals) {
        List<Interval> ans = new ArrayList<>();
        if(intervals.size()==0) return ans;
        //Collections.sort(intervals, (a,b)->(a.start-b.start));
        Collections.sort(intervals, new Comparator<Interval>(){
            @Override
            public int compare(Interval a, Interval b){
                return a.start - b.start;
            }
        });
        Interval pre = intervals.get(0);
        for(Interval in : intervals){
            if(in.start<=pre.end){
                pre.end = Math.max(pre.end, in.end);
            } else {
                ans.add(pre);
                pre = in;
            }
        }
        ans.add(pre);
        return ans;
    }
}