Weird non-determinism in 2197E1 (Interactive Graph)

While solving 2197E1 (Interactive Graph – Simple Version) in the recent Div 2, I ran into something really confusing, the same logic gave both AC and WA depending on tiny, irrelevant code changes.

I had multiple submissions within minutes:

Some Accepted, some WA on test 8 / 9 / 20 / 21 / 22 / 23

No logic changes. Same idea. Same queries.

The strangest example

This version ACs:

if (q == -1) { return; }

This version WAs:

if (q == -2) { return; } Logically, if I'm getting accepted, both branches are dead code.

Either the checker is non deterministic or there's some UB shenanigans going on.

Links to submissions

362583126 WA

362583031 AC

Darsh_Jain also tried testing it, and the following two submissions

362582232 WA

362581962 AC

And the difference between the two is only that we have added a comment in the first one!

FelixArg and sevlll777 pls help

EDIT: It was indeed UB :(, fixed now. PS: Always use #define _GLIBCXX_DEBUG kids.