Post order iterative - final code

PostOrderIterative_FinalCode.py

#The interviewee went over the example of tree, explained the algorithm to me. 
   1
  / \
 2   3
  \
   4
  /
 5

#After the visit:
#   1
#    
# 2   3
#  
#   4
#   
#  5  
#
#stack = 
#res = [5,4,2,3,1]

class Solution:
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        
        stack = [root]
        res = []
        
        while stack:
            s = stack[-1]
            if s.left:
                stack.append(s.left)
                s.left = None
            elif s.right:
                stack.append(s.right)
                s.right = None
            else:
                res.append(stack.pop())
         
        return res
        
#         stack1 = [root]
#         stack2 = []
#         while stack1:
#             node = stack1.pop()
#             if node:
#                 stack2.append(node.left)
#                 stack2.append(node.right)
        
#         return stack2[::-1]