[leetcode]Binary Tree Preorder Traversal

题目：

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

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

   1    \     2    /   3

return [1,2,3].

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

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

描述：二叉树的先序遍历，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 preorderTraversal(self, root):        if root == None: return []        stk, ans = [], []        p = root        while p!=None or len(stk)!=0:            if p == None:                p = stk.pop().right            else:                ans.append(p.val)                stk.append(p)                p = p.left        return ans