翻转二叉树

# 翻转一棵二叉树。 
# 
#  示例： 
# 
#  输入： 
# 
#       4
#    /   \
#   2     7
#  / \   / \
# 1   3 6   9 
# 
#  输出： 
# 
#       4
#    /   \
#   7     2
#  / \   / \
# 9   6 3   1 

方法：递归，子树处理

# leetcode submit region begin(Prohibit modification and deletion)
# 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: TreeNode) -> TreeNode:
        if not root:
            return
        root.left, root.right = self.invertTree(root.right), self.invertTree(root.left)
        return root
# leetcode submit region end(Prohibit modification and deletion)