An apex list doesn’t support contains(). It is only available for Sets and Maps. But since you don’t want to use Maps, you can use Sets to solve this problem. For that, you can define a new set like:
Set mySet = new Set();
Now you’ll need to use a method such as Set.addAll()to add all the elements of the lists to the set such as:
mySet.addAll(myList);.
You can now use the Set.contains() method to check the set for the element you’re looking for.