[swift] LeetCode 538. Convert BST to Greater Tree

Given 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 sum of all keys greater than the original key in BST.


Input: The root of a Binary Search Tree like this:
/ \
2 13

Output: The root of a Greater Tree like this:
/ \
20 13