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 invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
def dfs(root):
if not root:
return
tmp = root.left
root.left = root.right
root.right = tmp
dfs(root.left)
dfs(root.right)
dfs(root)
return root