def mergeOverlappingIntervals(intervals):
    # sort the intervals by startinf value.  O(nlogn)
    sortedIntervals = sorted(intervals, key = lambda x: x[0])
    
    mergedIntervals = []# space: O(n)
    currentInterval = sortedIntervals[0]#initialize the currentInterval as the first interval of the intervals
    mergedIntervals.append(currentInterval)#initialize the mergedIntervals as the first interval of the intervals
    
    for nextInterval in sortedIntervals:# for the first iteration, no update
        _, currentIntervalEnd = currentInterval
        nextIntervalStart, nextIntervalEnd = nextInterval
        
        if currentIntervalEnd >= nextIntervalStart: # overlap found, update the end value of interval
            currentInterval[1] = max(currentIntervalEnd, nextIntervalEnd)
        else: # no overlap, add the current(nextInterval) into the mergedIntervals array
            currentInterval = nextInterval
            mergedIntervals.append(currentInterval)
    return mergedIntervals