aka.Sohieb & Ibrahim_Mohamed are two accounts belongs to one person
Our cheater today is Sohieb Mohamed
you can see the rank of each account and notice the submission time for every problem
deader: http://codeforces.com/contest/550/standings/participant/4821774#p4821774
Sohieb_Mohamed: http://codeforces.com/contest/550/standings/participant/4832374#p4832374
and here are the submitted codes for both accounts
Ibrahim_Mohamed 'D': here
aka.Sohieb 'D': here
Ibrahim_Mohamed 'C': here
aka.Sohieb 'C': here
Ibrahim_Mohamed 'B': here
aka.Sohieb 'B': here
now I don't know how the cheating algorithm looks like but apparently it's not good enough specially in the two codes of problem 'D' which seems very identical.