sentence reverse - how to reverse the word in place?

SentenceReverse_30minutes.cs

using System;

class Solution
{
    public static char[] ReverseWords(char[] arr) // "a b cd"
    {
      if(arr == null || arr.Length == 0) // false 
      {
        return new char[0]; 
      }
      
      reverseInplace(arr, 0, arr.Length - 1); // 
      
      reverseEachWord(arr); // "dc b a"
      return arr; 
    }
 
    private static void reverseInplace(char[] arr, int start, int end)// 0, end = 5
    {
      var length = arr.Length;   // 6  
      
      while(start < end) // true
      {
        var tmp = arr[start]; // a
        arr[start] = arr[end]; // 'b'
        arr[end] = tmp; // 'a'
        
        start++;
        end--; 
      }
    }
  
    private static void reverseEachWord(char[] arr) // "dc b a"
    {
      const char space = ' '; 
      int length = arr.Length; // 6
      
      var start = 0; 
      var end = 0; 
      for(int i = 0; i < length; i++) // 'c'
      {
        var visit = arr[i]; // 'd'
        var isSpace = visit == space; // false
        
        if(isSpace)
        {
          continue; 
        }
        
        var isStart = i == 0 || arr[i-1] == space; // true
        var isEnd   = i == length - 1  || arr[i+1] == space; // false , true
        
        if(isStart)
        {
          start = i; // 0
        }
        
        if(isEnd)
        {
          end = i; // 1
          
          reverseInplace(arr, start, end); // "dc" - > "cd"
        }
      }
    }  
  
    static void Main(string[] args)
    {
      var reversed = ReverseWords("a b cd".ToCharArray()); 
      
      Console.WriteLine(new string(reversed)); 
      
    }
}

/*
"a b cd"
reverse the sentence -> "dc b a"  -> two pointer, in place
reverse each word, how do I do it?
"dc "
 ||
 start position - two checking index == 0 || arr[index - 1] == ' '
 end position - two checking   indx == lastChar || arr[index + 1] == ' '
 
eacH word, then reverse each word  - API 

*/