【LeetCode with Python】 Binary Tree Preorder Traversal

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原题页面：https://oj.leetcode.com/problems/binary-tree-preorder-traversal/
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本文地址：http://blog.csdn.net/nerv3x3/article/details/3465737

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?

非递归前序遍历二叉树。这里是用栈，先右孩子入栈，再左孩子入栈。隐约还记得如果是层次遍历，即需要用队列了。二叉树的非递归遍历中，记得后序遍历是最复杂的，需要用两个栈，以后再复习一下后序遍历。

class Solution:    # @param root, a tree node    # @return a list of integers    def preorderTraversal(self, root):        if None == root:            return [ ]        list = [ ]        stack = [ ]        cur = root        while True:            list.append(cur.val)            if None != cur.right:                stack.append(cur.right)            if None != cur.left:                stack.append(cur.left)            if len(stack) >= 1:                cur = stack.pop()            else:                break        return list