leetcode Binary Tree Postorder Traversal python

# 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]
        """
        res=[]
        stk=[]
        last=None
        if root == None:
            return res
        while root or len(stk) != 0:
            if root != None:
                stk.append(root)
                root=root.left
            else:
                tmpNode=stk.pop()
                stk.append(tmpNode)
                if tmpNode.right != None and last != tmpNode.right:
                    root=tmpNode.right
                else:
                    res.append(tmpNode.val)
                    stk.pop()
                    last=tmpNode
        return res