public static int stringConstruction(String s) {
        // goal: determine min cost of copying string s
        if(s.length() == 1) return 1;
        //this solution has o(n) time, o(n) space
        
        Set<Character> characterSet = new HashSet<>();
        //iterate over each char in s
        for(char c: s.toCharArray()){
            //if char isnt in char set, add to the total cost
            if(!characterSet.contains(c)){
                characterSet.add(c); //add char into char set
            }
        }
        return characterSet.size();
    }