using System;

using System.Linq;

​

public static class StringExtensions

{

    public static string ReverseWordOrder(this string input)

    {

        return string.Join(' ', input.Split(' ').Reverse());

    }

    public static string ReverseWordOrderOptimized(this string input)

    {

        Span<char> output = stackalloc char[input.Length];

        int lastSpace = input.Length - 1;

        int j = 0;

        for (int i = input.Length - 1; i >= -1; i--)

        {

            if (i == -1 || input[i] == ' ')

            {

                for (int k = i + 1; k <= lastSpace; k++)

                {

                    output[j++] = input[k];

                }

                if (i > 0)

                {

                    output[j++] = ' ';

                    lastSpace = i - 1;

                }

            }

        }

        return new string(output);

    }

}

​

class Program

{

    private static readonly (string Input, string Expected)[] Cases =

    {

        ("Simple test", "test Simple"),

        ("Lorem ipsum dolor sit amet", "amet sit dolor ipsum Lorem")

    };

    static void Main()

    {

        RunTests(StringExtensions.ReverseWordOrderOptimized);

    }

    static void RunTests(Func<string, string> method)

    {

        foreach (var test in Cases)

        {

            try

            {

                var actual = method(test.Input);

                var result = actual.Equals(test.Expected);

​

                if (result)

                {

                    Console.WriteLine($"\"{test.Input}\": OK");

                }

                else

                {

                    Console.WriteLine($"\"{test.Input}\": Fail (expected: \"{test.Expected}\", actual: \"{actual}\")");

                }

            }

            catch (Exception ex)

            {

                Console.WriteLine($"\"{test.Input}\": Fail with exception:\r\n{ex}");

            }

        }

    }

}