# 二叉树的前序遍历

<p>给你二叉树的根节点 <code>root</code> ，返回它节点值的 <strong>前序</strong><em> </em>遍历。</p>

<p> </p>

<p><strong>示例 1：</strong></p>
<img alt="" src="https://img-service.csdnimg.cn/img_convert/93c03b302d4108b3debe3c473af634ea.jpeg" style="width: 202px; height: 324px;" />
<pre>
<strong>输入：</strong>root = [1,null,2,3]
<strong>输出：</strong>[1,2,3]
</pre>

<p><strong>示例 2：</strong></p>

<pre>
<strong>输入：</strong>root = []
<strong>输出：</strong>[]
</pre>

<p><strong>示例 3：</strong></p>

<pre>
<strong>输入：</strong>root = [1]
<strong>输出：</strong>[1]
</pre>

<p><strong>示例 4：</strong></p>
<img alt="" src="https://img-service.csdnimg.cn/img_convert/428ab6a3e4c104cf4b41ad1a1a0a6f24.jpeg" style="width: 202px; height: 202px;" />
<pre>
<strong>输入：</strong>root = [1,2]
<strong>输出：</strong>[1,2]
</pre>

<p><strong>示例 5：</strong></p>
<img alt="" src="https://img-service.csdnimg.cn/img_convert/67437fd84487294f797443df319c90e9.jpeg" style="width: 202px; height: 202px;" />
<pre>
<strong>输入：</strong>root = [1,null,2]
<strong>输出：</strong>[1,2]
</pre>

<p> </p>

<p><strong>提示：</strong></p>

<ul>
	<li>树中节点数目在范围 <code>[0, 100]</code> 内</li>
	<li><code>-100 <= Node.val <= 100</code></li>
</ul>

<p> </p>

<p><strong>进阶：</strong>递归算法很简单，你可以通过迭代算法完成吗？</p>


## template

```java

public class TreeNode {
	int val;
	TreeNode left;
	TreeNode right;

	TreeNode(int x) {
		val = x;
	}
}

class Solution {
	public List<Integer> preorderTraversal(TreeNode root) {
		List<Integer> resultList = new ArrayList<>();
		if (root == null) {
			return resultList;
		}

		helper(resultList, root);

		return resultList;
	}

	public void helper(List<Integer> resultList, TreeNode root) {
		if (root == null)
			return;

		resultList.add(root.val);
		helper(resultList, root.left);
		helper(resultList, root.right);

	}

}

```

## 答案

```java

```

## 选项

### A

```java

```

### B

```java

```

### C

```java

```