This could be simply solved recurisivly.
# 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 root is None:
return root
root.left, root.right = root.right, root.left
self.invertTree(root.left)
self.invertTree(root.right)
return root