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- from itertools import chain # 这里要用itertool里的chain
- # 因为在python2里 range() 返回list
- # python3 里range 返回iterator,不能直接相加,要用chain方法
- for i in chain(range(n), range(n)[::-1]): # 这个写法可以实现一个for里双向遍历
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