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:
1 <= number of nodes <= 1000 <= Node.val <= 100root = [4,1,6,0,2,5,7,null,null,null,3,null,null,null,8][30,36,21,36,35,26,15,null,null,null,33,null,null,null,8]root = [0,null,1][1,null,1]