using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var result = Solution.TwoSum(new []{7,11,3,23,22,1,2}, 3);

        var leftIndex = result[0];

        var rightIndex = result[1];

        Console.WriteLine("{0} {1}", leftIndex, rightIndex);

    }

}

​

public class Solution

{

    private static int[]? CheckValue(

      (int, bool, int) source,

      IEnumerable<(int, bool, int)> comparable,

      int target

      ) 

    {

      var (value, equals, index) = source;

      return (comparable.Where(v => (value + v.Item1) == target).FirstOrDefault()) switch

      {

          (0, false, 0) => null,

          var (_, _, addIndex) => new []{ index, addIndex}

      };

    }

    private static int[] FindSolution(IReadOnlyList<(int, bool, int)> values, int target)

    {

        for (var i = 0; i < values.Count; i++)

        {

            var result = CheckValue(values[i], values.Skip(i+1), target);

            if(result == null) continue;

            return result;

        }

        throw new Exception("No values matched expected conditions.");

    }

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

    {

        // get members that are valid 

        var validMembers = nums

            .Select((v, i) => (v, v > target, v == target, i))

            .Where(vTuple => vTuple switch

            {

                (_, true, _, _) => false,

                _ => true

            }).Select(x => (x.Item1, x.Item3, x.Item4)).ToList();

​

        // if no members in set, can't continue

        if(validMembers.Count == 0) throw new Exception("No members in provided set can equal the target value");

        // check early exit case, target exists in set

        var targetInSet = validMembers.Where(x => x.Item2 == true);

        if (targetInSet.Count() > 0)

            return new int[] { -1, targetInSet.First().Item3 };

        else return FindSolution(validMembers, target);

    }

​

}