144\. Binary Tree Preorder Traversal

Easy

Given the `root` of a binary tree, return _the preorder traversal of its nodes' values_.

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/09/15/inorder_1.jpg)

**Input:** root = [1,null,2,3]

**Output:** [1,2,3] 

**Example 2:**

**Input:** root = []

**Output:** [] 

**Example 3:**

**Input:** root = [1]

**Output:** [1] 

**Example 4:**

![](https://assets.leetcode.com/uploads/2020/09/15/inorder_5.jpg)

**Input:** root = [1,2]

**Output:** [1,2] 

**Example 5:**

![](https://assets.leetcode.com/uploads/2020/09/15/inorder_4.jpg)

**Input:** root = [1,null,2]

**Output:** [1,2] 

**Constraints:**

*   The number of nodes in the tree is in the range `[0, 100]`.
*   `-100 <= Node.val <= 100`

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