Hint: If as per the rules CAT is "equal" to CTA, will it help of you change all CTA values to CAT in the list?...
Anyway. Did you start writing logical steps on paper like Winston suggested?
For example, if CAT and CTA are considered "equal", then you obviously won't be able to use String.equals(); so, how else do you think you might do that comparison?
List<String> ls = new ArrayList<String> ();
ls.add("CAT");
ls.add("MAT");
ls.add("TAM");
ls.add("CTA");
ls.add("AMT");