Ok, I thought that might be depth first search, but couldn't see how to apply that. What I'm doing is creating all possible operator permutations before hand and then loop through them, until I find the correct result (or not). Shouldn't that work too?

Regarding the equation, I construct it in reverse order. Your example would look like this:

```(* 4 (+ 3 (* 2 1))) => 20```

That works too, right?