希尔排序是插入排序的优化版，其算法表示如下：

def shell_sort(arr):
    if len(arr) == 1:
        return arr
    gap = round(len(arr) / 2)
    while gap > 0:
        # insert sort
        for i in range(gap, len(arr)):
            temp = arr[i]
            j = i - gap
            while j >= 0 and arr[j] > temp:
                arr[j + gap] = arr[j]
                j -= gap
            arr[j + gap] = temp
        gap = round(gap / 2)
    return arr