Given an array of integers where 1 ≤ a[i] ≤ n (n = size of array), 
some elements appear twice and others appear once.

Find all the elements of [1, n] inclusive that do not appear in this array.

Could you do it without extra space and in O(n) runtime? You may assume the returned list does not count as extra space.

面试碰到的英文Python题目，要求给一个数组，部分元素出现一次，部分元素出现两次，n为数组长度。找出数组长度中没有出现过的元素。于是尝试着写了一下，也不知道答案对了没？求大神支招并指点。

发现python比shell难呀

谁能告诉我如何快速学运维python呀

另外我觉得我的英文还要再好好练练，水平太次了。越学越糊涂真的是

因为之前有好几次学python都半途而废了，总是学到一半由于种种原因就放弃了，这次觉得不能在够放弃了。。。