镜像翻转二叉树

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
    root
end