using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
private static string ReverseString(string sentence)
{
List words = new List(sentence.Split(' ').ToList());
return words.Aggregate((a, b) => b + " " + a);
}
static void Main(string[] args)
{
string sentence = "this is a test";
string result = ReverseString(sentence);
Console.WriteLine(result);
}
}