[2% Bonus] Polish Hw#3 for next year

[taiya]

Hw3 is new this semester and the trace code is not super-polished yet. Create a super high-quality code + UI for use in future DGP courses, and redeem these bonus points! Only the first submission will be accepted

This is how you separate parts to be implemented from trace code:

#ifdef STRIP_CODE
        /// TASK: description of task goes here
#else
        /// C++ code goes here
#endif