Given an integer array, nums, of length $n$, where all the integers of nums are in the range $[1, n]$ and each integer appears once or twice, return an array of all the integers that appear twice.

Note: Write an algorithm that runs in $O(n)$ time and uses only constant auxiliary space, excluding the space occupied by the output.

Constraints:

• $n =$ nums.length

• $1 \leq n \leq 10^3$

• $1 \leq$ nums[i] $\leq n$

• Each element in nums appears once or twice.