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So for beginner/intermediate alg/data structures challenges with explanations and solutions I would recommend the following resources:

1. Read the Algorithm Design Manual.

2. Practice coding simple and then more advanced algorithms on sites like Coderbyte (aimed at beginners -> intermediate) and HackerRank (a bit more mathy).

3. Read as many algorithm explanations and code examples as you can on GeeksforGeeks.

4. Try and implement basic algorithms yourself like: shortest path, minimum spanning tree, DFS/BFS, tree traversals, different sorting algs, min/max heap, etc. and learn their running times.

* Also this article may be helpful for you: https://medium.com/coderbyte/how-to-get-good-at-algorithms-d...