human
Solve the following python programming problem given the following code:
# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        ans = []
        if root is None:
            return ans
        
        # iterative
        stack, lastVist = [root], root
        while stack:
            top = stack[-1]
            if (not top.left and not top.right) or top.left == lastVist or top.right == lastVist:
                lastVist = stack.pop()
                ans.append(lastVist.val)
            else:
                if top.right:
                    stack.append(top.right)
                if top.left:
                    stack.append(top.left)
        return ans
        
        
        # recursive
        return self.postorderTraversal(root.left) + self.postorderTraversal(root.right) + [root.val]
Given the above code, generate some markdown instructions for it.
assistant
#Here is the Python program you requested.

Binary Tree Postorder Traversal

https://leetcode.com/problems/binary-tree-postorder-traversal/#/description
Note: Recursive solution is trivial, could you do it iteratively?