144. Binary Tree Preorder Traversal

题目

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

Example :

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

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

题目大意

先根遍历一颗树。

解题思路

两种递归的实现方法，见代码。