using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

       int[] v = new int[] {2, 1, 8, 4, 7, 3};

       test(v, 3);

       test(v, 20);

       test(v, 1);

       test(v, 2);

       test(v, 7);

    }

​

    static void test(int[] v, int val)

    {

       bool output = find_sum_of_two(v, val);

       Console.WriteLine("exists(A, " + val + ") = " + output);

    }   

    static bool find_sum_of_two(int[] A, int val)

    {

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

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

       {

           if(found_values.Contains(val - A[i]))  

              return true;

​

           found_values.Add(A[i]);

       }

       return false;

    }   

}