public static List<Integer> closestNumbers(List<Integer> arr) {
        if(arr.size() == 2) return Arrays.asList(arr.get(0), arr.get(1));
        //goal: find pair with smallest abs diff
        //sort arr
        Collections.sort(arr);
        //init new min
        int min = arr.get(1) - arr.get(0);
        List<Integer> minIdices = new ArrayList<>();
        minIdices.add(arr.get(0));
        minIdices.add(arr.get(1));
        //compare adj elements
        for(int i = 2; i < arr.size(); i++){
            int diff = arr.get(i) - arr.get(i - 1);
            //if new min is found clear ans list
            if(diff < min){
                min = diff;
                minIdices.clear();
                minIdices.add(arr.get(i - 1));
                minIdices.add(arr.get(i));
            }
            else if(diff == min){
                minIdices.add(arr.get(i - 1));
                minIdices.add(arr.get(i));
            }
        }
        return minIdices;
    }