Post order iterative solution - find first element in post order traversal

PostOrderIterative.py

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)
            elif s.right:
                stack.append(s.right)
            else:
                res.append(stack.pop())
        
#         stack1 = [root]
#         stack2 = []
#         while stack1:
#             node = stack1.pop()
#             if node:
#                 stack2.append(node.left)
#                 stack2.append(node.right)
        
#         return stack2[::-1]