Reverse Words in C Sharp.NET

Lately I started some string manipulation in C Sharp, which you will see in the series to come. Please add your comments suggestions. These are optimized for performance

Input: Robert Frost
output is Frost Robert

  public static string ReverseWord(string s)
        {
            int length = s.Length;
            int i = 0;
            
            string[] splittedArray = s.Split(' ');
            StringBuilder sb = new StringBuilder();
            for (i = splittedArray.Length - 1; i >= 0; i--)
            {
                if (i != 0)
                    sb.Append(splittedArray[i] + ' ');
                else
                    sb.Append(splittedArray[i]); 

            }

            return sb.ToString();
        }

Here is another method to do the same using character array

public static string ReverseWordChar(string s)
        {
            int length = s.Length;
            int i = 0,j=0;
            StringBuilder sb= new StringBuilder(); 
            int startPos = length - 1;

            for (i = length-1; i >=0;i--)
            {
                if (s[i] == ' ')
                {
                    for (j = i+1; j <= startPos; j++)
                    {
                        sb.Append(s[j]);
                    }
                    startPos = i;
                    sb.Append(' ');
                }
            }

            for (j = 0; j < startPos; j++)
            {
                sb.Append(s[j]);
            }

            return sb.ToString();
        }