class Solution:
    def findDisappearedNumbers(self, nums: List[int]) -> List[int]:
        #TC: O(len(nums))
        #SC: O(1)

        #using negative sign in position of nums to indicate the existence of the elements

        for i in range(len(nums)):
            idx = abs(nums[i]) - 1
            nums[idx] = -1*abs(nums[idx])

        res = []
        for i in range(len(nums)):
            if nums[i] > 0:
                res.append(i+1)

        return res