Leetcode 56. Merge Intervals

https://leetcode.com/problems/merge-intervals/

Medium

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.

---

• 先排序，再挨个比较合并。Solution里对算法有反证，对于排序后的interval list，所有可以合并的interval，肯定是连续出现的。从而证明了算法里只需要把当前interval，跟前一个合并了的interval比较就行。
• https://leetcode.com/problems/merge-intervals/solution/

class Solution:
    def merge(self, intervals: List[List[int]]) -> List[List[int]]:
        if not intervals:
            return None
        
        intervals.sort(key=lambda x : x[0])
        
        merged = []
        
        for interval in intervals:
            # if the merged interval list is empty or 
            # the current interval does not overlap with the previous, 
            # simply append it.
            if not merged or merged[-1][1] < interval[0]:
                merged.append(interval)
            else:
            # otherwise, there is overlap, so merge the current and previous intervals.
                merged[-1][1] = max(merged[-1][1], interval[1])
        
        return merged