[leetcode]Binary Tree Postorder Traversal

题目：

Given a binary tree, return the postorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1    \     2    /   3

return [3,2,1].

Note: Recursive solution is trivial, could you do it iteratively?

链接：https://oj.leetcode.com/problems/binary-tree-postorder-traversal/

描述：二叉树的后序遍历，非递归

solution by python:

# Definition for a  binary tree node# class TreeNode:#     def __init__(self, x):#         self.val = x#         self.left = None#         self.right = Noneclass Solution:    # @param root, a tree node    # @return a list of integers    def postorderTraversal(self, root):        if not root: return []        stk,ans = [root,],[]        pre = root        while len(stk) != 0:            p = stk[-1]            if p.left==None and p.right==None or (p.left==pre or p.right==pre):                ans.append(p.val)                pre = stk.pop()            else:                if p.right != None: stk.append(p.right)                if p.left != None: stk.append(p.left)        return ans