---
description: Cyclic Sort
---

# Find All Duplicate Numbers in an Array

## Problem

Given an array of integers, 1 ≤ a\[i] ≤ n (n = size of array), some elements appear **twice** and others appear **once**.

Find all the elements that appear **twice** in this array.

{% hint style="info" %}
For example:

```
Input:
[4,3,2,7,8,2,3,1]

Output:
[2,3]
```
{% endhint %}

## Solution

```
class Solution:
    def findDuplicates(self, nums: List[int]) -> List[int]:
        
        i = 0
        res = []
        
        while i < len(nums):
            j = nums[i] - 1 #getting the index nums[i] needs to be at
            if nums[i] != nums[j]:
                nums[i], nums[j] = nums[j], nums[i]
            else:
                i+=1
        for i in range(len(nums)):
            if nums[i] != i+1:
                res.append(nums[i])
     
        return res
                
#Time: O(n)
#Space: O(1) ignoring the space required to return
```

