Codewars is an excellent service for strengthening your programming skills in Ruby, JavaScript, Java, Python, Clojure, CoffeeScript, C# , Haskell. Both algorithmic problems and practical problems related to design patterns are presented. After each solved problem, you can look at other solutions, sorted by rating depending on their beauty and use of the best practices in programming.
Leetcode is a site with algorithmic puzzles to prepare for technical interviews in large firms such as Google and Facebook. Near each task there is a discussion in which, one way or another, the solution to this task may appear. You can use various programming languages: C, C++, Java, Python, C# , JavaScript, Ruby, Bash.
C Puzzles provides you with programming puzzles specific to the C language (with all its quirks). Because many languages are C-like or directly derived from C, even if you don't know C as your first language, there's a good chance you'll understand these puzzles.
Quote from the main page: "Project Euler is a series of challenging math/programming problems that require mathematical intuition to solve them." The resource includes both relatively simple and truly complex puzzles.
Timus Online Judge is the largest archive of programming problems in Russia with an automatic checking system. The main source of tasks for the archive are competitions of the Ural Federal University, Ural Championships, Ural ACM ICPC quarter-finals, Petrozavodsk programming camps.
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