To contribute, create a new folder for each problem and work on your code there. PR's will not be accepted otherwise. 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.