using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var result = FindDuplicates(new int[]{4,3,2,7,8,2,3,1});

        Console.WriteLine(string.Join(",", result.ToArray()));

    }

    static IList<int> FindDuplicates(int[] nums) {

        IList<int> result = new List<int>();

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

        {

            var index = Math.Abs(nums[i]) - 1;

            if(nums[index] > 0)

                nums[index] *= -1;

            else

                result.Add(index + 1);

        }

        return result;

    }

}