class Solution:
    def findDisappearedNumbers(self, nums):
        # Pass 1: Mark existing numbers by negating values at the corresponding index
        for n in nums:
            # We use abs() because the number might have been negated already by a previous step
            idx = abs(n) - 1
            
            # Only negate if it's currently positive
            if nums[idx] > 0:
                nums[idx] *= -1

        # Pass 2: Collect indices that contain positive numbers
        res = []
        for i in range(len(nums)):
            if nums[i] > 0:
                # If the number at index i is positive, i+1 was missing
                res.append(i + 1)
                
        return res