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?

URL: https://leetcode.com/problems/binary-tree-postorder-traversal/

# 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]
        """
        if root == None:
            return []
        else:
            stack = []
            out_stack = []
            stack.append(root)

            while stack != []:
                current = stack.pop()
                out_stack.append(current.val)
                if current.left != None:
                    stack.append(current.left)
                if current.right != None:
                    stack.append(current.right)

            return out_stack[::-1]