LeetCode - 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].
The key to solve this problem is defining a Comparator first to sort the arraylist of Intevals. And then merge some intervals.
The take-away message from this problem is utilizing the advantage of sorted list/array.

 public ArrayList<Interval> merge(ArrayList<Interval> intervals) {
 
  if (intervals == null || intervals.size() <= 1)
   return intervals;
 
  // sort intervals by using self-defined Comparator
  Collections.sort(intervals, new IntervalComparator());
 
  ArrayList<Interval> result = new ArrayList<Interval>();
 
  Interval prev = intervals.get(0);
  for (int i = 1; i < intervals.size(); i++) {
   Interval curr = intervals.get(i);
 
   if (prev.end >= curr.start) {
    // merged case
    Interval merged = new Interval(prev.start, Math.max(prev.end, curr.end));
    prev = merged;
   } else {
    result.add(prev);
    prev = curr;
   }
  }
 
  result.add(prev);
 
  return result;
 }
class IntervalComparator implements Comparator<Interval> {
 public int compare(Interval i1, Interval i2) {
  return i1.start - i2.start;
 }
}

Read full article from LeetCode – Merge Intervals