using System;

using System.Text;

public class Program

{

    public static void Main(string[] args) {

        // you can write to stdout for debugging purposes, e.g.

        Console.WriteLine(ReverseByWordsV2("you can write to stdout for debugging purposes"));      

    }

​

    public static string ReverseByWordsV2(string input)

    {

        var output = new StringBuilder(input.Length);

        var wordStartIndex = input.Length - 1;

        for(var i = input.Length - 1; i >= 0; i--)

        {

            if (i == 0)

            {

                output.Append(input.AsSpan(0, wordStartIndex - i + 1));

                return output.ToString();

            }

​

            if (input[i] == ' ')

            {

                output

                    .Append(input.AsSpan(i + 1, wordStartIndex - i))

                    .Append(' ');

​

                wordStartIndex = i - 1;

            }

        }

​

        return output.ToString();

    }

}