Binary Search Tree to Greater Sum Tree

A BST visited in reverse inorder order, right then node then left, yields values from greatest to smallest. Maintain runningSum containing the original values of all nodes already visited, which are exactly the greater values. At each node, recurse right, add node.val to runningSum, assign node.val = runningSum, then recurse left. This transforms every node into itself plus all greater keys without needing a separate sorted array. Use a mutable accumulator shared by recursion or return it carefully; reset it before each independent invocation. If duplicate keys are permitted, clarify whether equal keys should all contribute, because standard BST duplicate policies vary. Test an empty tree, one node, a completely skewed tree, and values that can overflow the numeric type when summed. Every node is visited once, so time is O(n); recursion uses O(h) stack space where h is tree height, O(n) in the worst-case skew.