reverse words in a string.java

public class Solution {
    public String reverseWords(String s) {
        if (s == null)
            return "";
        String res = "";
        int len = s.length();
        for (int i = 0; i < len;) {
            String temp = "";
            while (i < len && s.charAt(i) == ' ')
                i++;
            if (i == len)
                break;
            while (i < len && s.charAt(i) != ' ')
                temp += s.charAt(i++);
            res = res.length() == 0? temp: temp + " " + res;
        }
        return res;
    }
}