binary-tree-postorder-traversal java code

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].
Note: Recursive solution is trivial, could you do it iteratively?

Thinking: using the depth traversal, backtracking method, the tree traversal sequence exists in the array, and return.

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */import java.util.ArrayList;public class Solution {    public ArrayList<Integer> postorderTraversal(TreeNode root) {        ArrayList<Integer> list = new ArrayList<Integer>();        if(root == null){            return list;        }        test(root,list);        return list;    }    public void test(TreeNode node,ArrayList<Integer> list){        if(node.left != null){            test(node.left,list);        }        if(node.right != null){            test(node.right,list);        }        list.add(node.val);    }}