面试题27. 二叉树的镜像

请完成一个函数，输入一个二叉树，该函数输出它的镜像。

例如输入：

     4
   /   \
  2     7
 / \   / \
1   3 6   9
镜像输出：

     4
   /   \
  7     2
 / \   / \
9   6 3   1

输入：root = [4,2,7,1,3,6,9]
输出：[4,7,2,9,6,3,1]

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


class Solution:
    def mirrorTree(self, root: TreeNode) -> TreeNode:
        if not root:
            return None
        

        right = self.mirrorTree(root.right)
        left = self.mirrorTree(root.left)
        root.right = left
        root.left = right
        return root