题目：
Invert a binary tree.
4
/
2 7
/ \ /
1 3 6 9
to
4
/
7 2
/ \ /
9 6 3 1
代码：递归处理即可
def invertTree(self, root):
"""
:type root: TreeNode
:rtype: TreeNode
"""
if root == None:
return None
root.left,root.right = root.right,root.left
self.invertTree(root.left)
self.invertTree(root.right)
return root
