## Flatten binary tree

I understand the solution but it's a bit difficult for me to come up on the spot. Need review.

```
from typing import Optional
# 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: Optional[TreeNode]) -> None:
self.dfs(root)
def dfs(self, root):
if not root:
return None
left = self.dfs(root.left)
right = self.dfs(root.right)
if left:
left.right = root.right
root.right = root.left
root.left = None
if right:
return right
elif left:
return left
return root
```