LeetCode #56 Merge Intervals

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Problem

Given a collection of intervals, merge all overlapping intervals.

Examples

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

Boundary Conditions

Solution

Method: Sort

Time Complexity: O(n log n)

Space Complexity: O(n)

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

class Solution:
    def merge(self, intervals):
        """
        :type intervals: List[Interval]
        :rtype: List[Interval]
        """
        if not intervals:
            return []

        left_pts = [interval.start for interval in intervals]
        right_pts = [interval.end for interval in intervals]
        ret = []
        left_pts.sort()
        right_pts.sort()

        start_ptr = 0
        end_ptr = 0
        shift = 1

        while start_ptr + shift < len(left_pts) and end_ptr < len(left_pts):
            if left_pts[start_ptr + shift] <= right_pts[end_ptr]:
                shift += 1
                end_ptr += 1
            else:
                ret.append(Interval(left_pts[start_ptr], right_pts[end_ptr]))
                end_ptr += 1
                start_ptr = end_ptr
                shift = 1

        ret.append(Interval(left_pts[start_ptr], right_pts[end_ptr]))

        return ret

or

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

class Solution:
    def merge(self, intervals):
        """
        :type intervals: List[Interval]
        :rtype: List[Interval]
        """
        out = []

        for i in sorted(intervals, key=lambda i: i.start):
            if out and i.start <= out[-1].end:
                out[-1].end = max(out[-1].end, i.end)
            else:
                out.append(i)

        return out