LeetCode—binary-tree-postorder-traversal（二叉树的后序遍历）—Java

题目描述：

Given a binary tree, return the postorder traversal of its nodes' values.

For example:
Given binary tree{1,#,2,3},

   1
    \
     2
    /
   3

return[3,2,1].

思路解析：

后序遍历：

• helper方法：递归先左子树，递归后右子树，插入到链表中
• 新建一个ArrayList<Integer> re =new ArrayList<Integer>();用于存放遍历顺序

代码：

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        ArrayList<Integer> re = new ArrayList<Integer>();
        if(root==null)
            return re;
        helper(root,re);
        return re;
    }
    public void helper(TreeNode root,ArrayList<Integer> re){//注意参数有两个哦
        if(root ==null)
            return ;
        helper(root.left,re);
        helper(root.right,re);
        re.add(root.val);
    }
}