using System;

public class Program

{

    public static void Main()

    {

        // Given an array of integers, write a function that finds all the duplicates in the array. 

        var A = new []{1,2,1,2}; 

        FindDuplicates(A); // expected: [1,2]

        var B = new []{3,3,3}; 

        //FindDuplicates(A); // expected: [3]

    }

    internal static int[] FindDuplicates(int[] arr)

    {

        int[] a=new int[arr.Length];

        for(int i=0 ;i<arr.Length;i++)

        {

            if (arr[ Math.Abs(arr[i])] >= 0) 

                arr[ Math.Abs(arr[i])] = -arr[ Math.Abs(arr[i])]; 

            else

                //a[i]=arr[i];

               Console.Write(Math.Abs(arr[i]) + " ");   

        }

        return a;

    }

}