Binary Search Tree to Greater Sum Tree

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.

Class TreeNode:
    val (int)
    left (TreeNode)
    right (TreeNode)

Input Format:

First Parameter - TreeNode root

Output Format:

Return the TreeNode.

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]
Ouptut:
    [1 null 1]

Constraints:

• The number of nodes in the tree is in the range [1,100].
• 0 <= node.val <= 100
• All the values in the tree are unique.
• Expected Time Complexity: O(N).
• Expected Space Complexity: O(N).