invert binary tree

# 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':
        node = root
        if node is None:
            return
        if node.left is not None:
            self.invertTree(node.left)
        temp = node.left
        if node.right is not None:
            self.invertTree(node.right)
        node.left = node.right
        node.right = temp
        return node