Mostly just brute forces through all possible combinations using basic dynamic programming and recursion. The only exception is it won't descend into multiplication path if the current number is not a multiple.
The basic solver (solvePN) generates a list of Polish Notation
solutions to avoid the need to figure out grouping. Then we run
polishNotationToInfix over those solutions to get a nice, normal, infix
string, which is eval-able.
There actually are tests. In jest.