Here are the sites I have experience with and my thoughts
- LeetCode is excellent because there are many problems and explanations. Furthermore the problems without explanations have answers and commented solutions in the discussion boards. It is the best online judge in my (limited) experience.
- Cracking the Coding Interview has many questions and detailed answers in Java.
- Data Structures and Algorithms Made Easy in Java is excellent. Sure there are many, many, many typos. But it has so many code samples for each of the data structures, explanations from worse (brute force) to best (optimized) along with advantages and disadvantages of the many possible solutions.
An honorable mention goes to Free Code Camp. The first time I ever had fun solving code challenges was at Free Code Camp. They did an amazing job of starting off with relatively easy challenges, then they raised the bar. Before I knew it I was doing difficult challenges, yet the hours flew by.