Merge Intervalsmedium

def merge(intervals: list[list[int]]) -> list[list[int]]:
    # TODO: sort by start, then merge overlapping intervals
    return []

print(merge([[1,3],[2,6],[8,10],[15,18]]))  # [[1,6],[8,10],[15,18]]
print(merge([[1,4],[4,5]]))                  # [[1,5]]