There will be folders named Week-1, Week-2 etc. To add your solution just submit a PR with you code to the corresponding week's folder. Please name your file like this: preferredname-4.lean if you are using Lean 4 or preferredname-3.lean if you are using Lean 3.
Using Lean 4's FFI is forbidden. (Worried people will use C for code optimization) Good Luck!