Top 13 links to interactively learn algorithm problem solving:

-> Hacker Rank https://www.hackerrank.com
-> Leet Code https://leetcode.com
-> Hacker Earth https://www.hackerearth.com
-> Exercism https://exercism.io
-> Code Wars https://www.codewars.com
-> Code In Game https://www.codingame.com
-> Check IO https://py.checkio.org
-> Algo Expert https://www.algoexpert.io
-> Top Coder https://www.topcoder.com
-> Code Forces http://codeforces.com
-> Code Chef https://www.codechef.com
-> TechGig https://www.techgig.com
-> SPOJ https://www.spoj.com