02. Common Elements

using System;

namespace CommonElements
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] arrOne = Console.ReadLine().Split();
            string[] arrTwo = Console.ReadLine().Split();
            string output = "";

            for (int i = 0; i < arrTwo.Length; i++)
            {
                for (int j = 0; j < arrOne.Length; j++)
                {
                    if (arrOne[j] == arrTwo[i])
                    {
                        output += arrTwo[i] + ' ';
                    }
                }
            }
            Console.WriteLine(output);
        }
    }
}

Решение с LINQ:

using System;
using System.Linq;

namespace CommonElements
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] arrOne = Console.ReadLine().Split();
            string[] arrTwo = Console.ReadLine().Split();

            arrTwo = arrTwo.Where(x => arrOne.Contains(x)).ToArray();

            Console.WriteLine(string.Join(" ", arrTwo));
        }
    }
}