Any hints or tests?

I have WA 30, what did you do to AC?

Match

Ok, I got AC after lots of WA 30 attempts by iterating over the input in the reversed order, which makes no sense to me. My current best guess is that test #30 is broken, so there are at least two correct answers and it accepts only one of them as correct.

Of course, my algo can delete edges in different order depending on the order in which it iterates over them, but I'm pretty sure it always removes the maximum number of edges.