题目：牛客网

解题思路：（来源：https://leetcode-cn.com/problems/palindrome-partitioning/solution/hui-su-you-hua-jia-liao-dong-tai-gui-hua-by-liweiw/）

搜索问题主要使用回溯法。

回溯法思考的步骤：

1、画递归树；

2、根据自己画的递归树编码。

import java.util.ArrayList;

public class Main {
	 public static boolean isPalindrome(String s) {
		    if (null == s || s.length() == 0) 
		    	return false;
		    int length = s.length();
		    int middle = length / 2;
		    for (int i = 0; i < middle;i++) {
		      if (s.charAt(i) != s.charAt(length - 1 - i)) {
		        return false;
		      }
		    }
		    return true;
	}
	
	public static ArrayList<ArrayList<String>> partition(String s) {
	    ArrayList<ArrayList<String>> res = new ArrayList<>();
	    ArrayList<String> list = new ArrayList<>();
	    partitionHepler(res, list, s);
	    return res;
	  }
	 
	  public static void partitionHepler(ArrayList<ArrayList<String>> res, ArrayList<String> list, String s) {
		//当字符串为空或长度为0时，add list
	    if (null == s || s.length() == 0) {
	      res.add(new ArrayList<>(list));
	      return;
	    }
	    int len = s.length();
	    for (int i = 1; i <= len; i++) {
	      String subStr = s.substring(0, i);
	      if (isPalindrome(subStr)) {
	        list.add(subStr);
	        partitionHepler(res, list, s.substring(i, len));
	        list.remove(list.size() - 1);
	      }
	    }
	  }
	 
	
	
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		System.out.println(partition("aab"));
	}

}