If one calls the code thus generated on a list of sorted intervals, the output is as expected, i.e. there are 2 groups of overlapping intervals,
First Group: [1,3] , [2,4]
Second Group: [5,7] , [6,8]
It works with an input of unsorted intervals (as expected). There are various tests that are not being done by the function e.g. for intervals of zero or negative lengths, duplicate intervals etc. The code's correctness is thus predicated on promises about the input.