Reverse Words in a String

//https://oj.leetcode.com/problems/reverse-words-in-a-string/
/*Reverse Words in a String Total Accepted: 21883 Total Submissions: 157273 My Submissions
Given an input string, reverse the string word by word.

For example,
Given s = "the sky is blue",
return "blue is sky the".

click to show clarification.
*/

import java.util.Stack;  


public class Solution {
    public String reverseWords(String s) {
        
        //ArrayList<String>  AL=new ArrayList<String>();
        StringBuilder   sb=new StringBuilder();
        Stack<Character> sk = new Stack<Character>();
        s.trim();
        
        for(int i=s.length()-1;i>=0;i--)
        {
          
            if(s.charAt(i)==' ')
            {
                 while(!sk.isEmpty())
                 {
                     sb=sb.append(sk.pop());
                 }   
             
             if(sb.length()>0&&sb.charAt(sb.length()-1)!=' ')
             {
                 sb.append(' ');
             }

                continue;
            }
           sk.push(s.charAt(i));
          
            
        }
        while(!sk.isEmpty())
         {
           sb=sb.append(sk.pop());
         }
         return sb.toString().trim();
        
    }
}