题目描述：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].

思路：根据区间的起始项先进行排序，然后两两比较，合并区间。

代码：

import java.util.*;
/**
 * 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 ArrayList<Interval> merge(ArrayList<Interval> intervals) {
        if(intervals == null || intervals.size() < 2){
            return intervals;
        }
        Collections.sort(intervals, new Comparator<Interval>(){
            @Override
            public int compare(Interval o1, Interval o2){
                return o1.start - o2.start;
            }
        });
        
        for(int i = 1; i < intervals.size(); i++){
            int preStart = intervals.get(i - 1).start;
            int preEnd = intervals.get(i - 1).end;
            int curStart = intervals.get(i).start;
            int curEnd = intervals.get(i).end;
            if(curStart <= preEnd){
                intervals.set(i, new Interval(preStart, Math.max(preEnd, curEnd)));
                intervals.set(i - 1, null);
            }
        }
        while(intervals.remove(null));
        return intervals;
    }
}