This is a great resource for learning data structures and algorithms. It's set up like the phase challenges that you're used to doing and have great, concise explanations of all the concepts.
- https://github.com/Devbootcamp/data-structures-and-algorithms-unit
- https://hackr.io/tutorials/learn-data-structures-algorithms
Or, if you learn better via video, this course:
Here are short explanations for Mergesort and Quicksort that are readable: