27. 二叉树的镜像

题目链接：https://leetcode-cn.com/problems/er-cha-shu-de-jing-xiang-lcof/
难度：简单

描述：
请完成一个函数，输入一个二叉树，该函数输出它的镜像。

题解

# 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:
        def recursion(root):
            if root is None:
                return
            root.left, root.right = root.right, root.left
            recursion(root.left)
            recursion(root.right)
        recursion(root)
        return root