using System;

public class Program

{

    public static void Main()

    {

        "Your string:".Dump();

        var s = Console.ReadLine();

        var c = s.ToCharArray();

        var result = c.ReverseWords();

        // Новая строка ниже создается только для удобного вывода результата

        // На этом этапе 

        (new string(result)).Dump();

    }   

}

​

static class CharArrayHelpers

{

    public static char[] ReverseChars(this char[] @this, long from = -1, long to = -1)

    {

        from = from > -1 ? from : 0;

        to = to > -1 ? to : @this.Length - 1;

        var middle = (to + from) / 2;

        char tmp;

        for (var ix = from; ix <= middle; ix++)

        {

            tmp = @this[ix];

            @this[ix] = @this[to - ix + from];

            @this[to - ix + from] = tmp;

        }

        return @this;

    }

​

    public static char[] ReverseWords(this char[] @this)

    {

        @this.ReverseChars();

​

        long ix = 0;

        long from;

        long to = 0;

        while (ix < @this.Length)

        {

            from = ix;

            while (ix < @this.Length && @this[ix] != ' ')

            {   

                ix++;

            }

​

            to = ix - 1;

            @this.ReverseChars(from, to);

            ix++;

        }

        return @this;

    }

}