Given the `root` of a binary tree and two integers `val` and `depth`, add a row of nodes with value `val` at the given depth `depth`.

Note that the `root` node is at depth `1`.

The adding rule is:

- Given the integer `depth`, for each not null tree node `cur` at the depth `depth - 1`, create two tree nodes with value `val` as `cur`'s left subtree root and right subtree root.
- `cur`'s original left subtree should be the left subtree of the new left subtree root.
- `cur`'s original right subtree should be the right subtree of the new right subtree root.
- If `depth == 1` that means there is no depth `depth - 1` at all, then create a tree node with value `val` as the new root of the whole original tree, and the original tree is the new root's left subtree.

Example 1:

    Input: root = [4,2,6,3,1,5], val = 1, depth = 2
    Output: [4,1,1,2,null,null,6,3,1,5]

Example 2:

    Input: root = [4,2,null,3,1], val = 1, depth = 3
    Output: [4,2,null,1,1,3,null,null,1]

Constraints:

- The number of nodes in the tree is in the range `[1, 104]`.
- The depth of the tree is in the range `[1, 104]`.
- `-100 <= Node.val <= 100`
- `-105 <= val <= 105`
- `1 <= depth <= the depth of tree + 1`