AtCoder

What AtCoder is

AtCoder is a competitive programming platform based in Japan. It runs rated online contests where you solve algorithmic problems against a timer, submit code in C++, Python, Java and many other languages, and get an instant verdict from the judge. Your results feed a rating, shown as a colour, that tracks your level over time.

Where some platforms are known for long stories and tricky wording, AtCoder is known for short, precise statements and carefully tested problems. That makes it a very good place to practise the part of competitive programming that matters most early on: turning a clear statement into a correct solution quickly.

The contest types

• ABC (AtCoder Beginner Contest): the regular contest for newer and middle-rated contestants. The contest list shows it rated for ratings up to 1999 and lasting 100 minutes. The first problems are gentle, the last ones are genuinely hard, so one ABC stretches a wide range of levels.
• ARC (AtCoder Regular Contest): harder, with more mathematical and constructive problems. Rated up to 2799.
• AGC (AtCoder Grand Contest): the hardest type, rated for everyone and aimed at the strongest contestants in the world.
• AHC (AtCoder Heuristic Contest): optimisation problems with no single correct answer, where you score points for better solutions. A different skill, and a fun one.

Is it worth it if I already use Codeforces?

Yes. Many contestants use both. Codeforces rounds reward speed across many problem styles, while an ABC gives you a steady ladder of problems in increasing difficulty with a clean statement each time. Doing an ABC most weekends and a Codeforces round during the week is a common, sensible routine.

The Educational DP Contest

AtCoder hosts the Educational DP Contest: 26 problems, A to Z, starting with "Frog 1" and covering most of the classic dynamic programming patterns, from one-dimensional DP and knapsack to DP on trees, bitmasks and digits. It is not rated and you can solve it whenever you like. For many contestants it is the single best set for learning DP properly.

What does it cost?

Nothing. Creating an account, entering contests and practising on past problems are free.

Common questions

Which language should I use? Any language the judge supports works for the early ABC problems. For the harder problems, C++ is the usual choice because of its speed and its standard library.

Can I use AI during a contest? Read AtCoder's current rules on generative AI before a rated contest. Rules differ between platforms and change over time.

How do I practise past contests? Open any past contest and solve its problems, or take it as a virtual contest. Solving the problem just above your level after each contest, known as upsolving, is where most improvement comes from.

Best for: anyone learning competitive programming who wants clean problems, a regular weekend contest and the best free set of DP problems there is.