using System;

using System.Collections.Generic;

​

​

public class Program

{

    public static void Main()

    {

        int[] nums = {1,2,3,6,7};

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

        List<int> result1 = new List<int>();

​

        HashSet<int> check = new HashSet<int>();

        foreach(int val in nums)

        {

            check.Add(val);

        }

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

        {

            if(!check.Contains(i))

            {

                result1.Add(i);

            }

        }

        Console.Write(String.Join(", ", result1)); ///// o(n) o(n)

​

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

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

             if (index < nums.Length && nums[index] > 0)

             nums[index] = -nums[index];

        }

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

            if (nums[i] > 0) result.Add(i+1);

        }

        Console.Write(String.Join(", ", result));  ///// o(n) o(1)

    }

}