public static List<Integer> icecreamParlor(int m, List<Integer> arr) {
        //goal: determine two indexes from arr that equal m when added together
        //use a map to store value and index
        Map<Integer, Integer> valueMap = new HashMap<>();
        for(int i = 0; i < arr.size(); i++){
            int price = arr.get(i); //current price
            int complement = m - price; // other price needed to equal m
            if(valueMap.containsKey(complement)){
                List<Integer> indices = Arrays.asList(valueMap.get(complement) + 1, i + 1); // 1 based index
                Collections.sort(indices);
                return indices;
            } else{
                valueMap.put(price, i); //value map didnt contain complement
            } 
        }
        return List.of(); //no valid pair was found
    }