I was working on the problem Xenia and Tree here: http://mirror.codeforces.com/problemset/problem/342/E

But the problem statement doesn't actually matter.

My first solution gets WA: http://mirror.codeforces.com/contest/342/submission/33140735

and my second gets AC: http://mirror.codeforces.com/contest/342/submission/33140905

The only difference between the two submissions that mattered is that I increased MAX_N from 100000 to 100005. This should not have changed anything, because I'm pretty sure my array sizes were all large enough. So why did this happen?