Brute Force

'''
Time: O(n)
Space: O(1)
'''
class Solution:
    def findDisappearedNumbers(self, nums: List[int]) -> List[int]:
        res = []
        set_nums = set(nums)
        n = len(nums)

        for i in range(1, n + 1):
            if i not in set_nums:
                res.append(i)
        return res