Invert Binary Tree

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"""
Invert a binary tree.
Example:
Input:
     4
   /   \
  2     7
 / \   / \
1   3 6   9
Output:
     4
   /   \
  7     2
 / \   / \
9   6 3   1
'''

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

"""

def invertTree(self, root):
    """
    :type root: TreeNode
    :rtype: TreeNode
    """
    if not root:
        return None

    right = self.invertTree(root.right)
    left = self.invertTree(root.left)

    root.right = left
    root.left = right

    return root