Problem - Find All Numbers Disappeared in an Array

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: nums = [4,3,2,7,8,2,3,1]. The numbers $5$ and $6$ are missing. Return [5, 6].

You'll use boolean flags (presence markers) to track which numbers appear. The constraint: do this without extra space (ignore the output array).