Solution: Find Two Numbers That Add Up to n

using System;
using System.Collections.Generic;
class Program
{
    /// <summary>
    /// Method to find two numbers that add up to n.
    /// </summary>
    /// <param name="arr">n array of integers.</param>
    /// <param name="n">The integer number n.</param>
    public static int[] FindSum(int[] arr, int n)
    {
        for (int i = 0; i < arr.Length; i++)
        {
            for (int j = 0; j < arr.Length; j++)
            {
                if (arr[i] + arr[j] == n && i != j)
                {
                    return new int[] { arr[i], arr[j] };
                }
            }
        }
        return new int[0];
    }
    public static void Main()
    {
        Console.WriteLine("[" + string.Join(", ", FindSum(new int[] { 1, 2, 3, 4 }, 5)) + "]");
    }
}

using System;
using System.Collections.Generic;
class Program
{
    static int BinarySearch(int[] arr, int item)
    {
        int first = 0;
        int last = arr.Length - 1;
        int found = -1;
        while (first <= last && found == -1)
        {
            int mid = (first + last) / 2;
            if (arr[mid] == item)
            {
                found = mid;
            }
            else
            {
                if (item < arr[mid])
                {
                    last = mid - 1;
                }
                else
                {
                    first = mid + 1;
                }
            }
        }
        return found;
    }
    /// <summary>
    /// Method to find two numbers that add up to n.
    /// </summary>
    /// <param name="arr">n array of integers.</param>
    /// <param name="n">The integer number n.</param>
    public static int[] FindSum(int[] arr, int n)
    {
        Array.Sort(arr);
        for (int i = 0; i < arr.Length; i++)
        {
            int index = BinarySearch(arr, n - arr[i]);
            if (index != -1 && index != i)
            {
                return new int[] { arr[i], arr[index] };
            }
        }
        return new int[0];
    }
    public static void Main()
    {
        Console.WriteLine("[" + string.Join(", ", FindSum(new int[] { 1, 2, 3, 4 }, 5)) + "]");
    }
}