using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(string.Join(",", FindDisappearedNumbers(new int[]{4, 3, 2, 7, 8, 2, 3, 1}).ToArray()));

    }

​

    private static IList<int> FindDisappearedNumbers(int[] nums)

    {

        var result = new List<int>();

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

        {

            int index = Math.Abs(nums[i]);

            if(nums[index - 1] > 0)

            {

                nums[index - 1] *= -1;

            }

        }

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

        {

            if(nums[i] > 0)

            {

                result.Add(i + 1);

            }

        }

        return result;

    }

    private static IList<int> FindDisappearedNumbers1(int[] nums)

    {

        var unique = new int[nums.Length + 1];

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

        {

            unique[nums[i]] = nums[i];

        }

​

        var result = new List<int>();

        for (int i = 1; i <= nums.Length; i++)

        {

            if (unique[i] == 0)

                result.Add(i);

        }

​

        return result;

    }

}