Merge Intervals

class Solution:
    def merge(self, intervals: List[List[int]]) -> List[List[int]]:
        
        intervals.sort(key=lambda x: x[0])
        res = [intervals[0]]

        for i in range(1, len(intervals)):
            # is overlapping
            if intervals[i][0] <= res[-1][1]:
                # merge and replace
                res[-1][1] = max(res[-1][1], intervals[i][1])
            else:
                res.append(intervals[i])

        return res