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