获得一个字符串的所有回文子串集合

import java.util.ArrayList;
public class Solution {
    //获得一个字符串的所有回文子串集合
    public ArrayList<ArrayList<String>> partition(String s) {
         ArrayList<ArrayList<String>>arr=new ArrayList<ArrayList<String>>();
         if(s==null||s.length()==0)
           return arr;
         ArrayList<String> list=new ArrayList<String>();
         dfs(0,s,list,arr);
         return arr;
    }
    //回溯法
    private void dfs(int index,String s,ArrayList<String> preList, ArrayList<ArrayList<String>>arr)
    {
        if(index==s.length()){
          arr.add(new ArrayList<String>(preList));
           return;
        }
        ArrayList<String> list = new ArrayList<String>(preList);
        for(int i=index;i!=s.length();i++){
           if(isHui(s,index,i))
           {
               list.add(s.substring(index,i+1));
               dfs(i+1,s,list,arr); //递归调用
               list.remove(list.size()-1);
           }
        }
    }
    //判断是否为回文串
    private boolean isHui(String s,int start,int end){
        while(start<=end){
            if(s.charAt(start++)!=s.charAt(end--))
            {
               return false;
            }
        }
        return true;  
    }
    public static void main(String[]args){
       System.out.println("Hello World！");
       String s="aaab";
       Solution sol=new Solution();
       System.out.println(sol.partition(s));

    }
}