SO50876993, C# - rextester

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var testSet = new string[]
            {
                "X", "O", "O",
                "O", "X", "O",
                "O", "X", "X"
            };
            var winner = GetWinner(testSet);
            if (winner != null)
            {
                Console.WriteLine(string.Format("{0} wins!", winner));
            }
        }
        
        
        private static string GetWinner(string[] grid)
        {
            var indexSets = new []
            {
                // Horizontal
                new [] { 0, 1, 2 },
                new [] { 3, 4, 5 },
                new [] { 6, 7, 8 },

// Vertical
                new [] { 0, 3, 6 },
                new [] { 1, 4, 7 },
                new [] { 2, 5, 8 },

// Diagonal
                new [] { 0, 4, 8 },
                new [] { 6, 4, 2 }
            };

foreach (var indices in indexSets)
            {
                var elements = indices.Select(i => grid[i]).ToArray();
                if (!string.IsNullOrEmpty(elements[0]) && elements[0] == elements[1] && elements[0] == elements[2])
                {
                    return elements[0];
                }
            }

return null;
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,17 sec, absolute running time: 0,09 sec, cpu time: 0,08 sec, average memory usage: 16 Mb, average nr of threads: 3