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# 0144. Binary Tree Preorder Traversal

<https://leetcode.com/problems/binary-tree-preorder-traversal>

## Description

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?

## ac

```java
```