Merge Intervals

Given a collection of intervals, merge all overlapping intervals.

Example:

Given intervals => merged intervals:

[                     [
  [1, 3],               [1, 6],
  [2, 6],      =>       [8, 10],
  [8, 10],              [15, 18]
  [15, 18]            ]
]

Challenge:

O(n log n) time and O(1) extra space.

Solution:

Sort all intervals based on the start first. Then, we could iterate through the interval list and compare current and last interval, if they are overlapped, extend the last interval; if they are not overlapped, save the last to result list and set current to last.

Code:

 /**
 * Definition of Interval:
 * public class Interval {
 *     int start, end;
 *     Interval(int start, int end) {
 *         this.start = start;
 *         this.end = end;
 *     }
 */
class Solution {
    /**
     * @param intervals, a collection of intervals
     * @return: A new sorted interval list.
     */
    public List<Interval> merge(List<Interval> intervals) {
        if (intervals == null || intervals.size() <= 1) {
            return intervals;
        }
        Collections.sort(intervals, new Comparator<Interval>() {
           public int compare(Interval a, Interval b) {
               return a.start - b.start;
           } 
        });
        List<Interval> res = new ArrayList<>();
        Interval last = intervals.get(0);
        for (int i = 1; i < intervals.size(); i++) {
            Interval cur = intervals.get(i);
            if (cur.start <= last.end) {
                last.end = Math.max(last.end, cur.end);
            } else {
                res.add(last);
                last = cur;
            }
        }
        res.add(last);
        return res;
    }


}