Leetcode (Python): 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?

Solution:

Recursive:

# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return a list of integers
    def postorderTraversal(self, root):
        solution = []
        self.postorderTraversalRec(root, solution)
        return solution
    
    def postorderTraversalRec(self, root, solution):
        if root == None:
            return
        self.postorderTraversalRec(root.left, solution)
        self.postorderTraversalRec(root.right, solution)
        solution.append(root.val)

Iterative:

# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return a list of integers
    def postorderTraversal(self, root):
        solution = []
        used = set()
        stack = []
        if root != None:
            stack.append(root)
        while len(stack)>0:
            node = stack.pop()
            if node in used:
                solution.append(node.val)
            else:
                used.add(node)
                stack.append(node)
                if node.right != None:
                    stack.append(node.right)
                if node.left != None:
                    stack.append(node.left)
        return solution