To contribute, create a new folder for each problem (if it doesn't exist) and work on your code there. Ex: Problem_1's solution should be in the same directory as the question. PR's will not be accepted otherwise. Make sure your GitHub username is at the top of your code in a comment. Keep the code clean, simple and well commented so that anyone in the future can understand easily. Any of these languages are fine:
C (
GNU GCC 6+), C++ (GNU GCC 6+), Java (Java SE 8+), Python (Python 3.6.5+)
If you want to use another language, create a new issue and a maintainer will respond.