Find All Duplicates in an Array (Leetcode 442)

Problem Link: https://leetcode.com/problems/find-all-duplicates-in-an-array/

class Solution:
    def findDuplicates(self, nums: List[int]) -> List[int]:
        
        
        # We are asked to do this in constant extra space.
        # So, something in-place must be done
        
        n=len(nums)
        ans=[]
        for i in range(n):
            
            idx=abs(nums[i])-1
            if(nums[idx]<0):
                ans.append(idx+1)
            nums[idx]=-1*nums[idx]
        return ans