public static List<Integer> quickSort(List<Integer> arr) {
        if(arr.size() == 1) return arr; //already sorted
        
        int pivot = arr.get(0); // pivot is always arr[0]
        //declare each subarray
        List<Integer> left = new ArrayList<>();
        List<Integer> right = new ArrayList<>();
        List<Integer> equal = new ArrayList<>();
        
        //partition the og arr
        for(int element : arr){
            if(element < pivot) left.add(element);
            else if (element > pivot) right.add(element);
            else equal.add(element);
        }
        
        //combine the subarrays
        List<Integer> combinedList = 
            Stream.of(left, equal, right)
            .flatMap(List::stream)
            .collect(Collectors.toList());
        return combinedList;
    }