Missing Numbers

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.

Example:

Input:
[4,3,2,7,8,2,3,1]

Output:
[5,6]

func findDisappearedNumbers(_ nums: [Int]) -> [Int] {
        
}