using System;

using System.Linq;

public class Program

{

    public static int[] TwoSum(int[] nums, int target)

    {

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

        {

            for (var j = 0; j < nums.Length; j++)

            {

                if (j == i) continue;

​

                if (nums[i] + nums[j] == target)

                {

                    return new[] { i, j };

                }

            }

        }

        return null;

    }

    public static int[] TwoSumWithLinq(int[] nums, int target)

    {

        return nums.SelectMany

        (

            (m, i) =>

            nums.Select

            (

                (n,j) => new { n, m, i, j }

            )

        )

        .Where

        (

            r => 

            r.i != r.j && r.m + r.n == target

        )

        .Select

        (

            r => new int[] { r.i, r.j }

        )

        .FirstOrDefault();

    }

    public static void Main()

    {

        var tests = new []

        {

            new { nums = new int[] { 1,2,3,4,5 }, target = 3 },

            new { nums = new int[] { 1,2,3,4,5 }, target = 6 },

            new { nums = new int[] { 1,2,3,4,5 }, target = 7 },

            new { nums = new int[] { 1,2,3,4,5 }, target = 9 }

        };

        foreach (var t in tests)

        {

            var oldMatch = TwoSum( t.nums, t.target );

            var newMatch = TwoSumWithLinq( t.nums, t.target );

            Console.WriteLine

            (

                "{0},{1} == {2},{3} (matching target {4} over array {{{5}}})", 

                oldMatch[0], 

                oldMatch[1], 

                newMatch[0], 

                newMatch[1],

                t.target,

                string.Join(",", t.nums)

            );

        }

    }

}