[Leetcode] 140. Work Break II

Given a string s and a dictionary of words dict, add spaces in s to construct a sentence where each word is a valid dictionary word.
Return all such possible sentences.
For example, givens =  "catsanddog" ,dict =  ["cat", "cats", "and", "sand", "dog"] .
A solution is  ["cats and dog", "cat sand dog"] .
これはやはりNPコースで、forサイクルにbacktrackingを追加しますが、これはタイムアウトします.

import java.util.ArrayList;
public class Solution {
    public List wordBreak(String s, Set dict) {
        ArrayList result = new ArrayList();
        if(s == null || s.length() == 0) return result;
        ArrayList list = new ArrayList();
        helper(s, dict, result, 0, list);
        return result;
    }
    private void helper(String s, Set dict, ArrayList result, int start, ArrayList list){
        if(start == s.length()){
            StringBuffer sb = new StringBuffer();
            for(int i = 0; i < list.size() - 1; i++){
                sb.append(list.get(i) + " ");
            }
            sb.append(list.get(list.size() - 1));
            result.add(sb.toString());
            return;
        }
        for(int i = start; i < s.length(); i++){
            String cur = s.substring(start, i + 1);
            if(dict.contains(cur)){
                list.add(cur);
                helper(s, dict, result, i + 1, list);
                list.remove(list.size() - 1);
            }
        }
    }
}