HashMap<String,List<String>> map = new HashMap<String,List<String>>();

    public List<String> wordBreak(String s, Set<String> wordDict) {
        List<String> res = new ArrayList<String>();
        if(s == null || s.length() == 0) {
            return res;
        }
        if(map.containsKey(s)) {
            return map.get(s);
        }
        if(wordDict.contains(s)) {
            res.add(s);
        }
        for(int i = s.length() - 1; i >= 0 ; i--) {
            String t = s.substring(i);
            if(wordDict.contains(t)) {
                String remain = s.substring(0, i);
                List<String> temp = wordBreak(remain , wordDict);
                if(temp != null) {
                    for(String word : temp) {
                        res.add(word + " " + t);
                    }
                }
            }
        }
        map.put(s , res);
        return res;
    }