string s = "this is a test";
string[] words = s.Split(' ');
StringBuilder sb = new StringBuilder();
for (int i = words.Length - 1; i >= 0; i--)
{
for (int j = words[i].Length -1; j >= 0;j--)
{
sb.Append(words[i][j]);
}
sb.Append(" ");
}
Console.WriteLine(sb);