这题简单，直接递归左右节点对掉即可。注意判空操作，如果传入为空，则什么也不做。
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def invertTree(self, root):
"""
:type root: TreeNode
:rtype: TreeNode
"""
if not root:
return
if root.left or root.right:
root.left, root.right = root.right, root.left
self.invertTree(root.left)
self.invertTree(root.right)
return root
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