Once I solved an Advent of Code problem, I felt like the problem wasn't general enough, so I solved the more general version as well. I like programming to the point of doing imaginary homework, then writing myself some extra credit and doing that as well. Way too much for my own good.

The point is that solving a new problem is interesting. Solving a problem you already know exactly how to solve isn't interesting and isn't even intellectual exercise. I would gain approximately zero from writing a new hash table from scratch whenever I needed one instead of just using std::map.

Problem solving absolutely is a muscle and it's use it or lose it, but you don't train problem solving by solving the same problem over and over.