思路很简单，出口是空节点，先翻转子节点，再返回。

TreeNode* invertTree(TreeNode* root) {        if (root == nullptr){        return root;    }        invertTree(root->left);    invertTree(root->right);    swap(root->left, root->right);    return root;}

def invert_tree(root)    return root if root == nil    invert_tree(root.left)    invert_tree(root.right)    root.left, root.right = root.right, root.left    rootend