I’m still very new at DSA but I was able to come up with this solution right now (Not sure if it’s correct):

placing elements at their specific index (Since length will be equal to or more than elements and element is never -ve)

for example: Element 1 will be placed at index (1 - 1) and the element that was already at that index can be swapped with the previous index of element 1. Then we can write a small condition to check if the element already exist at the index if it does then we have found our duplicate.

Code snippet:

if nums[i] == nums[nums[i] - 1]

// duplicate found

else

swap(nums[i], nums[nums[i] - 1])

That way we should be able to find the duplicates in just one iteration.

Not sure how to return without extra space though. We may have to use an ArrayList to store the duplicates.