Binary Search Tree to Greater Sum Tree

Problem Description

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/

Solution (Go)

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func bstToGst(r *TreeNode) *TreeNode {
	var sum int

	var bst2Gst func(root *TreeNode)
	bst2Gst = func(root *TreeNode) {
		if root.Right != nil {
			bst2Gst(root.Right)
		}
		root.Val += sum
		sum = root.Val
		if root.Left != nil {
			bst2Gst(root.Left)
		}
	}
	bst2Gst(r)

	return r
}