Given the `root` of a Binary Search Tree (BST), convert it to a Greater Tree such that every key of the original BST is changed to the original key plus the sum of all keys greater than the original key in BST.

As a reminder, a binary search tree is a tree that satisfies these constraints:

- The left subtree of a node contains only nodes with keys less than the node's key.
- The right subtree of a node contains only nodes with keys greater than the node's key.
- Both the left and right subtrees must also be binary search trees.

Example 1:

    Input: root = [4,1,6,0,2,5,7,null,null,null,3,null,null,null,8]
    Output: [30,36,21,36,35,26,15,null,null,null,33,null,null,null,8]

Example 2:

    Input: root = [0,null,1]
    Output: [1,null,1]

Constraints:

- The number of nodes in the tree is in the range `[0, 104]`.
- `-104 <= Node.val <= 104`
- All the values in the tree are unique.
- `root` is guaranteed to be a valid binary search tree.

Note: This question is the same as 1038: https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/