Sentence reverse - need to read the problem carefully, reverse twice and the word goes back to original order.

sentenceReverse_practice.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]; 
      }
      
      var length = arr.Length; // 6 
      reverseWordInplace(arr, 0 , length - 1); // "dc b a"
      
      // reverse each word                   
      int start = 0; 
      
      for(int index = 0; index < length; index ++) //"dc b a"
      {
         var visit = arr[index];          
         var isNonSpacer  = visit != ' '; // true
        
         var isFirst = isNonSpacer && (index == 0 || arr[index - 1] == ' ');  // d, false 
         if(isFirst) // true
         {
           start = index; // 0
         }
        
         var isLast  = isNonSpacer && (index == (length - 1) || arr[index + 1] == ' '); // false, true
        
        if(isLast)
        {
          reverseWordInplace(arr, start, index); //0, 1           
        }                        
      }
      
      return arr; 
    }

    private static void reverseWordInplace(char[] arr, int start, int end )// "a b cd" 0, 5 -> "dc b a"
    {            
      while(start < end) // 
      {
        var temp = arr[start]; 
        arr[start] = arr[end]; 
        arr[end] = temp; 
        start ++; // 0 -> 1
        end   --; // 5 -> 4
      }
    }
  
    static void Main(string[] args)
    {

    }
}

// "a b cd" -> "cd b a" -> each word -> reverse each word -> "cd"
// inplace all the time -> reverse the string 
// last word -> first word, every word the word is reversed, reverse again
// instead of API - > by myself 
// time complexity -> empty char ' ' delimiter 
// space complexity - O(1)

Leetcode 151 - reverse words in a string, medium level algorithm. Need to go over leetcode discussion and figure out what to learn. Nov. 9, 2017