[LeetCode] Reverse Words in a String

Problem:

Given an input string, reverse the string word by word.
For example,
Given s = "the sky is blue",
return "blue is sky the".

Java Code:

public class Solution {
    public String reverseWords(String s) {
        if (s == null || s.length() == 0)
            return s;
        
        ArrayList<String> wordList = new ArrayList<String>();
        
        StringBuilder subString = new StringBuilder();
        StringBuilder result = new StringBuilder();
        int index = 0;
        for (int i = s.length() - 1; i >= 0; i--) {
            if (s.charAt(i) != ' ')
                subString.append(s.charAt(i));
            else if (subString.length() != 0){
                if (result.length() != 0)
                    result.append(" ");
                result.append(subString.reverse());
                subString = new StringBuilder();
                index = i - 1;
            }
        }
        if (subString.length() != 0) {
            if (result.length() != 0)
                result.append(" ");
            result.append(subString.reverse());
        }
            
        
        return result.toString();
    }
}