| Codeforces |
Highly recommended for its high quality content and also for a large variety of problems. Virtual Contests are another great way to improve your speed and accuracy. Highly active community is a plus point. |
| AtCoder |
Highly recommended for its Beginner Contests. Known for it’s quality problems. Do give it a try if you’re getting started with CP. |
| Codechef |
Has been very active in improving the Indian CP culture through it’s various initiatives. It’s Long Challenge is recommended. |
| CSES Problem Set |
Developed by Antti Laaksonen, it contains mostly educational problems. |
| HackerRank |
A good user-friendly website where you can easily learn different langauges and practice problems. It is recommended for the intial stages (for eg :- learning a language) of your CP journey. |
| HackerEarth |
A great place to learn theory and practice it. Organises three monthly contests including a long challenge. |
| SPOJ |
Provides a great pool of different variety of problems. Preferred for its Classical and Standard problems. |