二叉树的非递归后序遍历, 还是此文, ^_^

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return a list of integers
    def postorderTraversal(self, root):
        stack = []; res = []
        pre = None
        while root or stack:
            if root:
                stack.append(root)
                root = root.left
            elif pre == stack[-1].right:
                pre = stack.pop()
                res.append(pre.val)
            else:
                root = stack[-1].right
                pre = None
        return res