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# 用户需求：写一个快速排序算法并测试
def quick_sort(arr):
    if len(arr) <= 1:
        return arr
    pivot = arr[len(arr) // 2]
    left = [x for x in arr if x < pivot]
    middle = [x for x in arr if x == pivot]
    right = [x for x in arr if x > pivot]
    return quick_sort(left) + middle + quick_sort(right)

# 运行测试结果：
# [1, 3, 5, 7, 9] 运行成功 (0.02s)