There is no reason to store the non odd occuring numbers. Doing so requires you to loop through again and find the odds. It's much slower than deleting a key if it's already in the object

As you have likely surmised it's because it takes a pass through to essentially remove all non parens first. One could simply ignore anything that isn't a paren.

This is too many loops. You're looping through the whole array twice for every item. You should be able to do this with two loops max. Maybe less?