# Python3

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """

        # approach: use iteration

        stack = []
        result = []
        current = root

        while current or len(stack):
            # push right children if it exists then current root
            # then down to left children
            while current:
                if current.right:
                    stack.append(current.right)
                stack.append(current)
                current = current.left

            node = stack.pop()

            # for current root, its right children haven’t be handled
            # then handle right children first and then current root
            # by popping right children out and pushing current root back
            if len(stack) and node.right == stack[-1]:
                current = stack.pop()
                stack.append(node)
            else:
                result.append(node.val)

        return result