public static void quickSort(int[] arr, int low, int high) {

        if (arr == null || arr.length == 0) return;

        if (low >= high) return;

        //pick the pivot
        int middle = low + (high - low) / 2;
        int pivot = arr[middle];

        //make left < pivot and right > pivot
        int i = low, j = high;
        while (i <= j) {
            while (arr[i] < pivot) i++;
            while (arr[j] > pivot) j--;

            if (i <= j) {
                int temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
                i++;
                j--;
            }
        }

        //recursively sort two sub parts
        if (low < j) quickSort(arr, low, j);
        if (high > i) quickSort(arr, i, high);
    }