invertBinaryTree

# 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: Optional[TreeNode]):
            if root == None:
                return(root)
            
            
            root_tmp = root.left
            root.left = root.right
            root.right = root_tmp
            dfs(root.left)
            dfs(root.right)

            return(root)

        dfs(root)
        return(root)