1038 - Binary Search Tree to Greater Sum Tree\.

Medium

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 [1, 100].

• 0 <= Node.val <= 100

• All the values in the tree are unique.

Note: This question is the same as 538: https://leetcode.com/problems/convert-bst-to-greater-tree/