I experienced a strange WA on 576C - Points on Plane.
Compare these two submissions: 13000332 and 13000388. They are exactly the same, except that one submission had comparators <= and >=, while the other had comparators < and >. There shouldn't be any undefined behaviour as far as I can see, so I'm wondering why the WA in the first submission.
This would have been very frustrating had it been during a contest... Is there anything I'm missing here?
http://en.cppreference.com/w/cpp/algorithm/sort
http://en.cppreference.com/w/cpp/concept/Compare
Ahhh, this is great to know. Thank you very much!