56. Merge Intervals

https://leetcode.com/problems/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].

/**
 * 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; }
 * }
 */
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) {
  if (intervals == null || intervals.size() <= 1) {
   return intervals;
  }
 
  // sort intervals by using self-defined Comparator
  Collections.sort(intervals, new IntervalComparator());
 
  List<Interval> result = new ArrayList<Interval>();
 
  Interval prev = intervals.get(0);
  for (int i = 1; i < intervals.size(); i++) {
   Interval curr = intervals.get(i);
 
   if (hasIntersection(prev, curr)) {
    // merged case
    Interval merged = merge(prev, curr);
    prev = merged;
   } else {
    result.add(prev);
    prev = curr;
   }
  }
 
  result.add(prev);
 
  return result;
 }
 
 public Interval merge(Interval a, Interval b) {
        return new Interval(Math.min(a.start, b.start), Math.max(a.end, b.end));
    }
    
    
    public boolean hasIntersection(Interval a, Interval b) {
        //a.end intersects with [b.start, b.end] or vice versa
        return (a.end >= b.start && a.end <= b.end)
        || (b.end >= a.start && b.end <= a.end); 
    }
}

class IntervalComparator implements Comparator<Interval> {
 public int compare(Interval i1, Interval i2) {
  return i1.start - i2.start;
 }
}