27. 二叉树的镜像

public TreeNode Mirror(TreeNode root) {
    if (root == null)
        return root;
    swap(root);
    Mirror(root.left);
    Mirror(root.right);
    return root;
}

private void swap(TreeNode root) {
    TreeNode t = root.left;
    root.left = root.right;
    root.right = t;
}
​