翻转二叉树

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def solve():
    def invertTree(root):
        if not root:
            return None
        root.left, root.right = root.right, root.left
        invertTree(root.left)
        invertTree(root.right)
        return root
    root = TreeNode(4, TreeNode(2, TreeNode(1), TreeNode(3)), TreeNode(7, TreeNode(6), TreeNode(9)))
    result = invertTree(root)
    print(result.val)