Exercism
exercism.org
A free platform for improving coding skills through practice exercises in over 70 programming languages, with optional human mentorship from volunteers.
Why it is useful
The mentorship model is what distinguishes Exercism — real humans review your code and give structured feedback, not automated hints. This produces a qualitatively different learning experience than automated platforms. Coverage of 70+ languages means it is useful for learning an obscure language like Erlang or Clojure as much as for improving Python fundamentals. Completely free.
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