Coding challenge sites are a great way for you to sharpen your skills or measure against your past progress. There are a variety of coding challenges, from simple puzzlers that force you to think outside the box to timed challenges that force you to get the correct answer within a specified amount of time.

The timed challenges can also be used by sites like TopCoders or Toptal when vetting potential freelancers.

Here are some of my favorites in no particular order:

• Exercism - unique blend of exercises mixed with code reviews and mentoring
• CodeWars - coding katas with other developers
• Project Euler - a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve
• CodinGame - an actual game that forces you to write code to move troops, etc.
• TopCoder - challenge site with recruiting
• CoderByte - challenge site, language courses and interview help as well
• HackerRank (30 Days of Code) - 30 Day coding challenge, unlocking new challenges as you finish previous day
• CodeChef - Challenge site with leaderboards and other courses
• LeetCode - Practice questions, challenges and interview help
• SPOJ - Sphere Online Judge (got me ), seems more focused on algorithm improvement
• 25 Puzzles and Brain Teasers - Great list of puzzles and brain teasers

Cryptography challenges

• CryptoPals - good hands-on intro to crypto
• id0-RSA - another good hands-on intro to crypto
• Micro Corruption - in order to learn more you need to break their security (I've heard this is very challenging)

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