问题描述：
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.(找到这种数：包含在1~n之间，但是在数组中空缺的数。要是没明白我说的，那就看下面的例子，一下就明白了)
Could you do it without extra space and in O(n) runtime? You may assume the returned list does not count as extra space.
示例：
Example:
Input: [4,3,2,7,8,2,3,1] Output: [5,6]
问题来源：Find All Numbers Disappeared in an Array (具体地址：https://leetcode.com/problems/find-all-numbers-disappeared-in-an-array/#/description)
思路：其实和Find All Duplicates in an Array 这道题是一类问题，只是问题换了个说法而已，你可以清楚的看出来代码其实都是差不多的Find All Duplicates in an Array代码部分。
代码：
