It's not about code optimization, but about using the correct algorithm (don't mistake "using the correct time complexity" with "optimization" which suppose you're always keeping the same time complexity).

All in all, you need to find a better alogrithm, not to optimize your code.

EDIT: I just saw you use Shell. Don't forget to give that kind of indication in your message. Especially when it's a niche language like that x) )