Solution

using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
public static void Sort(int[,] arr)
 {

for (int i = 0; i < arr.GetLength(0); i++)
{
      for (int j = arr.GetLength(1) - 1; j > 0; j--)
     {
          for (int k = 0; k < j; k++) {
            if (arr[i, k] > arr[i, k + 1]) {
            int myTemp = arr[i, k];
            arr[i, k] = arr[i, k + 1];
            arr[i, k + 1] = myTemp;
       }
     }
   }
 }
}

public static List<int> GetSolution(int[,] P)
{
        var Q = new int[P.GetLength(0), P.GetLength(1)];

        for (int i = 0; i < P.GetLength(0); i += 2)
        {
            for (int j = 0; j < P.GetLength(1); j++)
            {
                Q[i,j] = P[i,j];
            }
        }

        Sort(Q);

        var arr = new int[Q.GetLength(0) * Q.GetLength(1)];
        int arrIndex = 0;

        for (int i = 0; i < Q.GetLength(0); i++)
        {
            for (int j = 0; j < Q.GetLength(1); j++)
            {
                arr[arrIndex] = Q[i,j];
                arrIndex++;
            }
        }

    var list = new List<int>();

        for (int i = 0; i < arr.Length; i++)
        {
            if (arr[i] == i)
            {
                list.Add(i);
            }
        }

    return list;
}

    public static void Main(String[] args) {
        var P = new int[,]
        {
            {
                1,1,1,2
            },
            {
                2,3,3,4
            },
            {
                4,4,4,4
            },
            {
                1,2,3,1
            }
        };

        var result = GetSolution(P);
        var resStr = String.Join(", ", result);

        Console.WriteLine(resStr);
    }
    }
}