(http://projecteuler.com) (http://codility.com) (http://mirror.codeforces.com) (http://contest.bayan.ir) (http://usaco.com) (http://acm.sgu.ru) CROATIAN OLYMPIAD IN INFORMATICS : (http://http://hsin.hr/coci/) (http://acm.timus.ru) (https://www.topcoder.com/) CEOICENTRAL-EUROPEAN OLYMPIAD IN INFORMATICS : (http://ceoi.inf.elte.hu/)