BOL

There are many websites available but i suggest you to start with the basic and then advance.

Follow the procedure

1. Try to focus on Data Structure and Algorithm from GeeksforGeeks | A computer science portal for geeks . First try to understand the algorithm from the above mentioned .IDeserve - Unique learning platform to enhance algorithmic skills.(video lectures).Answers to all your technical interview questions .Tutorial Horizon
2. Now,you are well versed with knowledge of Data Structure and Algorithm.The only thing you need is to Apply it Topcoder is repeatable, on-demand crowdsourcing, done right, at scale. Programming Competition,Programming Contest,Online Computer Programming HackerRank .HackerEarth - Programming challenges and Developer jobs
3. The above mentioned websites have a lot of variations of questions on Algorithms and Data Structure .so you learn and apply it.