Reverse Words in a String in C#

I am here with another example on how to reverse the words in a string or sentence?

I have tried with an approach. But I would welcome other approaches as well, please share.

////// Complexity: O(n) + O(n)/2 ~ O(n)
public static string ReverseWordsInString(string str)
{
    string reverse = ReverseString(str);
    string temp = "";
    foreach(string s in reverse.Split(' '))
    {
        temp += ReverseString(s) + ' ';
    }
    return temp;
}
private static string ReverseString(string str)
{
    char[] chars = str.ToCharArray();
    int j = str.Length - 1;
    for (int i = 0; i < str.Length / 2; i++)
    {
        char c = chars[i];
        chars[i] = chars[j];
        chars[j] = c;
        j--;
    }
    return new string(chars);
}