140. Word Break II

   /*
    * 140. Word Break II
    * 2016-5-23 by Mingyang
    * 这个题目用backtracking也可以，注意这里的string累积的形式
    * newItem就是积累的String
    */
    public ArrayList<String> wordBreak(String s, Set<String> dict) {
        ArrayList<String> res = new ArrayList<String>();
        if (s == null || s.length() == 0)
            return res;
        helper(s, dict, 0, "", res);
        return res;
    }
    private void helper(String s, Set<String> dict, int start, String item,
            ArrayList<String> res) {
        if (start >= s.length()) {
            res.add(item);
            return;
        }
        StringBuilder str = new StringBuilder();
        for (int i = start; i < s.length(); i++) {
            str.append(s.charAt(i));
            if (dict.contains(str.toString())) {
                String newItem = item.length() > 0 ? (item + " " + str
                        .toString()) : str.toString();
                helper(s, dict, i + 1, newItem, res);
            }
        }
    }