Binary Tree Preorder Traversal@leetcode

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?

用stack做，res存结果，不要混淆，那么我们要的是res的顺序是preorder, 那么stack是一个缓冲，root, left, right, 那么stack是root, right, left, 用一个while循环来模拟．