/// <summary>
/// Function to reverse sentence word by word
/// </summary>
/// <param name="sentence">string (sentence)</param>
/// <returns>reversed string by word</returns>
public string ReverseWordByWord(string sentence)
{
	char[] str = sentence.ToCharArray();
 
	str = Reverse(str, 0, str.Length - 1);
 
	int start = 0, end = 0;
	for (int i = 0; i < str.Length; i++)
	{
		if (str[i] == ' ')
		{
			end = i - 1;
			str = Reverse(str, start, end);
			start = i + 1;
		}
	}
 
	//For Last word
	end = str.Length - 1;
	str = Reverse(str, start, end);
 
	return new string(str);
}

/// <summary>
/// Helper method to reverse string character by character.
/// </summary>
/// <param name="str">string to be reversed</param>
/// <param name="start">start index</param>
/// <param name="end">end index</param>
/// <returns>reversed string</returns>
public char[] Reverse(char[] str, int start, int end)
{
	while (end > start)
	{
		str[start] ^= str[end];
		str[end] ^= str[start];
		str[start] ^= str[end];
		end--;
		start++;
	}
	return str;
}

string output=ReverseWordByWord("This is an eXclusiveMinds article");
Console.WriteLine(output);

//input: This is an eXclusiveMinds article
article eXclusiveMinds an is This