I was working on the problem Xenia and Tree here: http://mirror.codeforces.com/problemset/problem/342/E But the actual problem statement doesn't matter.

I am unsure why 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.