Reverse Words in a String

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

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

Update (2015-02-12):
For C programmers: Try to solve it in-place in O(1) space.

 

import java.util.ArrayList;
import java.util.Collections;

public class Solution {
    public String reverseWords(String s) {
    	if (s == null || s.length() == 0) {
    		return s;
    	}
    	String[] split = s.split(" ");
    	if (split == null || split.length == 0) {
    		return "";
    	}
    	ArrayList<String> arrayList = new ArrayList<String>();
    	for (int i = 0; i < split.length; i++) {
			if (!split[i].equals("")) {
				arrayList.add(split[i]);
			}
		}
    	Collections.reverse(arrayList);
    	String res = new String();
    	for (int i = 0; i < arrayList.size() - 1; i++) {
			res += arrayList.get(i) + " ";
		}
    	res += arrayList.get(arrayList.size()-1);
    	return res;
    }
}