Java for Leetcode 145 Binary Tree postorder traversal

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] .

Post-the-loop traversal, left subtree → right sub-tree → root node

The non-recursive implementation of the pre-sequence traversal requires a counter, the method is to rewrite a class to inherit TreeNode, Honghuiyu textbook "Data structure: Solution and Extension" P113 detailed introduction, here slightly. The recursive Java implementation is as follows:

    Public list<integer> postordertraversal (TreeNode root) {    list<integer> List = new Arraylist<integer > (); if (root = null) return List;list.addall (Postordertraversal (Root.left)); List.addall (Postordertraversal ( Root.right)); List.add (root.val); return list;    }

Java for Leetcode 145 Binary Tree postorder traversal