# \[144]\[中等] 二叉树的前序遍历

## 题目描述

[144. 二叉树的前序遍历](https://leetcode-cn.com/problems/binary-tree-preorder-traversal/)

给定一个二叉树，返回它的 前序 遍历。

示例:

```
输入: [1,null,2,3]  
   1
    \
     2
    /
   3

输出: [1,2,3]
```

进阶: 递归算法很简单，你可以通过迭代算法完成吗？

## 解题思路

### 递归

```python
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        res = []

        def dfs(node):
            if node is None:
                return

            res.append(node.val)
            dfs(node.left)
            dfs(node.right)
        dfs(root)
        return res
```

### 迭代

```python
class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        if root is None:
            return []
        res, stack = [], [root]
        while stack:
            node = stack.pop(-1)

            res.append(node.val)
            if node.right is not None:
                stack.append(node.right)
            if node.left is not None:
                stack.append(node.left)
        return res
```


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