I conduct interviews on interviewing.io so I'd just like to add some clarification on this advice. You don't get unlimited interviews so it's best used for interview practice, not coding practice. If you have trouble with the technical parts (sorting, trees, etc), you can learn that on your own from books/leetcode. Once you're confident in your coding and CS fundamentals, use interviewing.io for the aspects of interviewing that you can't learn from leetcode: gathering requirements from the interviewer, explaining your thought process, staying calm under pressure, time management, etc.
