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?
http://www.cnblogs.com/dolphin0520/archive/2011/08/25/2153720.html
http://yucoding.blogspot.com/2013/12/leetcode-question-binary-tree-preorder.html
[LeetCode] Binary Tree Preorder Traversal 二叉树的先序遍历
原文：http://www.cnblogs.com/grandyang/p/4146981.html