In my hundreds of submissions, this is my first time encountering this Singular Iterator Runtime Error. In my submission 86437246, after passing the first few cases and when it encounters a relatively bigger input, the program exits with a RE. After looking at the diagnostic hints, I think it has something to do with singular iterators ( which is sort of equivalent to uninitialized pointers ), though I couldn't pinpoint where. But if I were to make a guess, I would say vector< pair< int, int > > which[1050] should be the source of the problem as after using some alternatives for it, I managed to get my solution accepted (86437933).

It would be very helpful if someone can throw some light on where I went wrong, so that I could avoid this in my future submissions.

Thanks a lot!