希尔排序

给定一个系数，所有元素按照系数间隔进行插入排序

def shell_sort(a):
    step = len(a) // 2
    while step:
        for i in range(step, len(a)):
            while i-step >= 0:
                if a[i] < a[i - step]:
                    a[i], a[i - step] = a[i - step], a[i]
                    i -= step
                else:
                    break

        step //= 2

    return a