Merge Intervals

MergeIntervals.java

/**
 * 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; }
 * }
 */
public class Solution {
    public ArrayList<Interval> merge(ArrayList<Interval> intervals) {
        ArrayList<Interval> res = new ArrayList<Interval>();
        if(intervals == null || intervals.size() == 0) 
            return res;
        Collections.sort(intervals, new Comparator<Interval>(){
            public int compare(Interval a, Interval b) {
                return a.start > b.start ? 1 : (a.start == b.start ? 0 : -1);
            }
        });
        for(int i = 0; i < intervals.size(); i++) {
            Interval cur = intervals.get(i);
            if(res.isEmpty()) {
                res.add(cur);
            } else {
                Interval last = res.get(res.size() - 1);
                if(last.end >= cur.start) {
                    // merge 
                    last.end = Math.max(last.end, cur.end);
                } else {
                    res.add(cur);
                }
            }
        }
        return res;
    }
    
}