LC448. Find All Numbers Disappeared in an Array

class Solution:
    def findDisappearedNumbers(self, nums: List[int]) -> List[int]:
        """ O(N)/O(1)
        two loops 
        1. iterate throught each num, mark nums[num-1] as negtive 
        2. loop through the array,  the positions where numbers are positive is the missing numbers 
        
        instead of using negtive sign to mark numbers at position i already existed, can use +N as offset and check if nums[i] < N 
        """
        res = []
        n = len(nums)
        for i in range(n):
            x = abs(nums[i])-1
            nums[x] = -1 * abs(nums[x])# here need to multiply by abs value of nums[x], else original negtive value will be cancel out to positive
        
        for i in range(n):
            if nums[i] > 0:
                res.append(i+1)
        return res