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].

MergeIntervals.java

/**
 * 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; }
 * }
 */
 
import java.util.*;
 
public class Solution {
    
    public ArrayList<Interval> merge(ArrayList<Interval> intervals) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(intervals.size() == 0)
            return intervals;
        if(intervals.size() == 1)
            return intervals;
        
        Collections.sort(intervals, new IntervalComparator());
        
        Interval first = intervals.get(0);
        int start = first.start;
        int end = first.end;
        
        ArrayList<Interval> result = new ArrayList<Interval>();
        
        for(int i = 1; i < intervals.size(); i++){
            Interval current = intervals.get(i);
            if(current.start <= end){
                end = Math.max(current.end, end);
            }else{
                result.add(new Interval(start, end));
                start = current.start;
                end = current.end;
            }
            
        }
        
        result.add(new Interval(start, end));
        
        return result;
        
    }
}


class IntervalComparator implements Comparator{
        public int compare(Object o1, Object o2){
            Interval i1 = (Interval)o1;
            Interval i2 = (Interval)o2;
            return i1.start - i2.start;
        }
}