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].

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

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    private Stack<TreeNode> stack = new Stack<>();private void leftDown(TreeNode node){while (node != null) {stack.push(node);node = node.left;}}public ArrayList<Integer> postorderTraversal(TreeNode root) {ArrayList<Integer> res = new ArrayList<>();        leftDown(root);        TreeNode last = null;        while (!stack.isEmpty()) {TreeNode node = stack.peek();TreeNode right = node.right;if (right != null && node.right != last) {leftDown(right);}else{stack.pop();res.add(node.val);last = node;}}        return res;    }}