Laurent_R:

Yes, the task was too simple to really need much in the way of a data structure. I contemplated using a single hash with a string or bitmap for +/-, and also an HoA where different array slots would handle different symbols (+, -). In the end, I went with a HoH because it should be easy to understand, and didn't need any gymnastics. Two hashes works fine, too, it just didn't occur to me.

...roboticus

When your only tool is a hammer, all problems look like your thumb.