56. Merge Interval

Given a collection of intervals, merge all overlapping intervals.

Example 1:

Input:
 [[1,3],[2,6],[8,10],[15,18]]

Output:
 [[1,6],[8,10],[15,18]]

Explanation:
 Since intervals [1,3] and [2,6] overlaps, merge them into [1,6].

Example 2:

Input:
 [[1,4],[4,5]]

Output:
 [[1,5]]

Explanation:
 Intervals [1,4] and [4,5] are considered overlapping.

Thoughts:

1. Sort the interval list according the start point

2. record end, for loop check whether the current interval's start time is after the record end; if it is, merge the interval by maxing the end time; if it is not, insert the interval of recorded start and end time and update them to start another interval to be merged with current start and end time.

Code Java O(nlogn): Java8 lambda comparator

/**
 * 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 Solution {
    public List<Interval> merge(List<Interval> intervals) {
        if(intervals.size() <= 1) return intervals;

        intervals.sort((i1, i2)->Integer.compare(i1.start, i2.start)); // java8 lambda

        int start = intervals.get(0).start, end =intervals.get(0).end;
        List<Interval> results = new LinkedList <Interval>();
        for(Interval i: intervals){
            if(i.start <= end){
             end = Math.max(i.end, end);           // merging  
            }
            else{
              results.add(new Interval(start, end));   // disjoint: first adding existing merged intervals, then update start, end
              start = i.start; end = i.end; // update start, end
            }
        }

        results.add(new Interval(start, end));
        return results;
    }
}

Code Python

# Definition for an interval.
# class Interval(object):
#     def __init__(self, s=0, e=0):
#         self.start = s
#         self.end = e

class Solution(object):
    def merge(self, intervals):
        """
        :type intervals: List[Interval]
        :rtype: List[Interval]
        """
        intervals =sorted(intervals, key=lambda x:x.start)
        results = []

        l = len(intervals)
        i = 0
        while i < l:
            s = intervals[i].start
            e = intervals[i].end
            j = i + 1
            while j < l and e >= intervals[j].start:
                e = max(e, intervals[j].end)
                j += 1
            results.append(Interval(s,e))
            i = j
        return results