145.py

#!/usr/bin/env python
# coding=utf-8

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

# recursive version



class Solution1(object):
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        ans = []
        def dfs(node):
            if not node:
                return
            dfs(node.left)
            dfs(node.right)
            ans.append(node.val)
            return
        
        dfs(root)
        return ans

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
# iterative version
class Solution(object):
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        ans, stack = [], [root]
        stack = [root]
        while stack:
            top = stack.pop()
            if top:
                ans.append(top.val)
                stack.append(top.left)
                stack.append(top.right)
        
        return ans[::-1]

if __name__ == '__main__':
    s = Solution()
    print s.postorderTraversal([])