Merge Intervals

def merge(intervals):
    intervals.sort(key=lambda i: i[0])
    output = [intervals[0]]
    
    for start, end in intervals[1:]:
        lastEnd = output[-1][1]
        
        if start <= lastEnd:
            output[-1][1] = max(lastEnd, end)
        else:
            output.append([start, end])
            
    return output