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