Find All Numbers Disappeared in an Array

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.

Latest Source Code:
Github: MissingNumbers.java

Output:

Array: [4, 3, 2, 7, 8, 2, 3, 1]
Missing number: [5, 6]
Array: [1, 1]
Missing number: [2]
Array: [9, 9, 4, 10, 8, 5, 2, 2, 7, 7]
Missing number: [1, 3, 6]