翻轉一棵二叉樹。

輸入：

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

輸出：

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

代碼如下：

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

class Solution:
    def invertTree(self, root: TreeNode) -> TreeNode:
        if not root:
            return
        root.left, root.right = root.right, root.left
        self.invertTree(root.left)
        self.invertTree(root.right)

參考：
https://leetcode-cn.com/problems/invert-binary-tree/