Leetcode offers a variety of programming challenge questions to help you both improve your algorithmic thinking and language knowledge. It supports a huge number of languages, and it has questions ranging from easy to downright impossible. Though much of the website is accessible free of charge, there is a rather pricey paywall behind some of the problems. However, several of the questions that came up in my interviews with actual companies in industry were on the website, so there’s no denying it’s a brilliant resource.