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:
4 (30)
/ \
/ \
1 (36) 6 (21)
/ \ / \
/ \ / \
0 (36) 2 (35) 5 (26) 7 (15)
\ \
3 (33) 8 (8)
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, 10^{4}]
. 10^{4} <= Node.val <= 10^{4}
 All the values in the tree are unique.
root
is guaranteed to be a valid binary search tree.

