public static string ReverseWords(string s)

{

 var sb = new StringBuilder(s);

 Reverse(sb, 0, sb.Length - 1);

 int start = 0;

 int end = 0;

 for (int i = 1; i < sb.Length; i++)

 {

  if (sb[i - 1] != ' ')

  {

   end++;

   if (sb[i] != ' ')

   {

    continue;

   }

  }

  else

  {

    start = i;

    end = i;

  }

  Reverse(sb, start, end - 1);

  start = i;

  end = i;

 }

 Reverse(sb, start, end);

 return sb.ToString();

}

private static void Reverse(StringBuilder sb, int start, int end)

{

 while (start < end)

 {

  var c = sb[end];

  sb[end] = sb[start];

  sb[start] = c;

  end--;

  start++;

 }

}

Some sample test cases:
string s = "The quick brown fox jumped over the trot";Debug.Assert(s == ReverseWords(ReverseWords(s))) ;Debug.Assert("" == ReverseWords(ReverseWords("")));Debug.Assert("abc" == ReverseWords(ReverseWords("abc")));Debug.Assert("ab bc" == ReverseWords(ReverseWords("ab bc")));Debug.Assert(" ab " == ReverseWords(ReverseWords(" ab ")));Debug.Assert("ab " == ReverseWords(ReverseWords("ab ")));Debug.Assert(" ab" == ReverseWords(ReverseWords(" ab")));