I encountered an unusual runtime error during today's Div 2 round on problem D. http://mirror.codeforces.com/contest/706/problem/D

I implemented a simple trie, and received runtime error on test 1. http://mirror.codeforces.com/contest/706/submission/19803196

After some more submissions, I got to this: http://mirror.codeforces.com/contest/706/submission/19805829

With a couple of custom invocations, I narrowed the problem down to function mx. Implementing it a bit differently, I got AC. http://mirror.codeforces.com/contest/706/submission/19808959

After the contest, I tried submiting my code to several other sites: SPOJ, UVa, ideone. I also ran the code locally both on Windows and Linux. Out of all of these, CodeForces was the only platform on which the program crashed.

Is there a bug in the compiler, or am I missing something?