Different from HackerRank Codility does not provide good tests or allow you to run your code and see testing failures, so you need to remember to add test cases to make sure your code also solves the corner cases. This is the hardest portion of the interview because the coding challenge is often not real world and focus on algorithmic performance and matrix traversal, if you prepare on HackerRank and Codility itself you should be able to pass this portion.