0442-find-all-duplicates-in-an-array.py

class Solution:
    def findDuplicates(self, nums: List[int]) -> List[int]:
        res = []

        for n in nums:
            n = abs(n)
            if nums[n - 1] < 0:
                res.append(n)
            nums[n - 1] = -nums[n - 1]
        
        return res