word-break Java code

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, given
s =”catsanddog”,
dict =[“cat”, “cats”, “and”, “sand”, “dog”].
A solution is[“cats and dog”, “cat sand dog”].

import java.util.ArrayList;import java.util.HashSet;import java.util.Set;public class Solution {    public  ArrayList<String> wordBreak(String s, Set<String> dict) {         ArrayList<String> lists = new ArrayList<String>();         ArrayList<String> strings = new ArrayList<String>();         int len = s.length();         dfs(s,len,dict,lists,strings);        return lists;    }    public  void dfs(String s,int len,Set<String> dict, ArrayList<String> lists,ArrayList<String> strings) {        if(len<=0){            int size = strings.size()-1;            StringBuilder sb = new StringBuilder();            for(;size>=0;size--){                sb.append(strings.get(size));                if(size!=0)                    sb.append(" ");            }                 lists.add(sb.toString());        }        for(int k = len-1;k>=0;k--){            if(dict.contains(s.substring(k,len))){                strings.add(s.substring(k,len));                dfs(s,k,dict,lists,strings);                strings.remove(strings.size()-1);            }        }    }}