public static void QuickSort(int[] arr, int l, int r) {
        if (r <= l) return;
        int pivot = partition(arr, l, r);
        QuickSort(arr, l, pivot - 1);
        QuickSort(arr, pivot + 1, r);

    }

    static int partition(int[] arr, int l, int r) {
    	// pivot表示当前标杆，counter用于计数比arr[pivot]小的值
        int pivot = r, counter = l;
        // 比arr[pivot]小的值都放在最左边
        for (int i = l; i < r; ++i) {
            if (arr[i] < arr[pivot]) {
                int tmp = arr[i];
                arr[i] = arr[counter];
                arr[counter] = tmp;
                ++ counter;
            }
        }
        // 处理标杆
        int tmp = arr[counter];
        arr[counter] = arr[pivot];
        arr[pivot] = tmp;
        return counter;
    }