Challenge #23: 🔢 Find the missing numbers

The elves are working on a system to verify children's gift lists 👧👦. However, some lists are incomplete and numbers are missing!

Your task is to write a function that, given an array of numbers, finds all the numbers that are missing between 1 and n (where n is the size of the array or the highest number in the array).

Keep in mind that:

• Numbers may appear more than once and others may be missing
• The array always contains positive integers
• Counting always starts from 1

findMissingNumbers([1, 2, 4, 6])
// [3, 5]

findMissingNumbers([4, 8, 7, 2])
// [1, 3, 5, 6]

findMissingNumbers([3, 2, 1, 1])
// []

findDisappearedNumbers([5, 5, 5, 3, 3, 2, 1])
// [4]