140. Word Break II

class Solution {
    public List<String> wordBreak(String s, List<String> wordDict) {
        Map<String,List<String>> map=new HashMap<String,List<String>>();
        wordBreak(s, map, wordDict);
        return map.get(s);
    }
    private void wordBreak(String s, Map<String,List<String>> map, List<String> wordDict){
        if(map.containsKey(s))
            return;
        List<String> list=new ArrayList<String>();
        if(s.length()==0)
        {
            list.add("");
            map.put(s,list);
            return;
        }
        for(String w:wordDict)
            if(s.startsWith(w))
            {
                String str=s.substring(w.length());
                wordBreak(str,map,wordDict);
                List<String> l=map.get(str);
                for(String sentence:l)
                    list.add(w+(sentence.length()!=0?" ":"")+sentence);
            }
        map.put(s,list);
    }
}