> For the complete documentation index, see [llms.txt](https://blessbingo.gitbook.io/garnet/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://blessbingo.gitbook.io/garnet/suan-fa/shu/114-er-cha-shu-zhan-kai-wei-lian-biao.md).

# \[114]\[中等]\[DFS] 二叉树展开为链表

## 题目描述

[114. 二叉树展开为链表](https://leetcode-cn.com/problems/flatten-binary-tree-to-linked-list/)

给定一个二叉树，原地将它展开为一个单链表。

例如，给定二叉树

```
    1
   / \
  2   5
 / \   \
3   4   6
```

将其展开为：

```
1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6
```

## 解题思路

### 寻找前驱节点

空间复杂度$$O(1)$$.

如果一个节点的左子节点不为空, 则该节点的左子树中的最后一个节点被访问之后, 该节点的右子节点被访问.

* 左子树中最后一个被访问的节点是左子树中的最右边的节点, 是右子树根节点的前驱结点
* 当前树的根节点是左子树根节点的前驱结点

递归地将树展开, 然后将左右子树根节点与它们的前驱结点相连.

```python
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right


class Solution:
    def flatten(self, root: TreeNode) -> None:
        """
        Do not return anything, modify root in-place instead.
        """
        def dfs(node):
            if node is None:
                return

            raw_left, raw_right = node.left, node.right
            left = dfs(raw_left)
            right = dfs(raw_right)
            if left is not None:
                node.right = raw_left
                node.left = None
                node = left
            if right is not None:
                node.right = raw_right
                node = right
            return node

        dfs(root)
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://blessbingo.gitbook.io/garnet/suan-fa/shu/114-er-cha-shu-zhan-kai-wei-lian-biao.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.