I am pretty sure that during Educational Codeforces Round 100 (Rated for Div. 2), buihoatpt2k3 copied code from huyhoangk36chv.

buihoatpt2k3's behavior is quite suspicious in general — he (?) submitted all his code during the last 30 minutes, and his code is heavily obfuscated.

We see, however, that after running the preprocessor (gcc -E) on the code, we get code that is strikingly similar to that of huyhoangk36chv.

For example, take 1463A - Dungeon. Here's huyhoangk36chv's code: 101515478. And here's the de-obfuscated code from buihoatpt2k3: 101775980.

Same deal for problem D: 101562763, 101776426.

I think this is pretty good evidence that these users cheated and copied code from each other, and as such, call for their participation in the contest to be voided.