https://www.hackerrank.com/ – HackerRank http://www.spoj.com/ – Sphere Online Judge http://hkoi.org/en/past-problems/ – Hong Kong Olympiad in Informatics Problems http://hkoi.org/en/final-event-solutions/ – Hong Kong Olympiad in Informatics Solutions https://uva.onlinejudge.org/ – UVA Online Judge http://poj.org/ – Peking University Online Judge http://codeforces.com/ – CodeForces https://www.topcoder.com/ – TopCoder https://www.codechef.com/ – CodeChef http://rosettacode.org/wiki/Category:Programming_Tasks – Rosetta Code https://projecteuler.net/ – Project Euler http://www.codeabbey.com/ – CodeAbbey […]