Reverse Words in a String

题目：

Given an input string, reverse the string word by word.

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

public String reverseWords(String s) {
        if(null==s) return null;
        StringBuilder sb = new StringBuilder("") ;
        String[] vals = s.trim().split(" ");
        for(int i=vals.length-1;i>=0;i--){
                
            if(!vals[i].equals("")){
                if(i<vals.length-1)
                    sb.append(" ");
                sb.append(vals[i]);
            }
        }
        return sb.toString();
    }