C++ Solution to Coding Challenge 448. Find All Numbers Disappeared in an Array

Problem statement

Given an array nums of n integers where nums[i] is in the range [1, n], return an array of all the integers in the range [1, n] that do not appear in nums.

Example 1

Input: nums = [4,3,2,7,8,2,3,1]
Output: [5,6]