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
Python3
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def bstToGst(self, root: TreeNode) -> TreeNode:
curr = 0
def go(node):
nonlocal curr
if not node: return None
node.right = go(node.right)
curr += node.val
node.val = curr
node.left = go(node.left)
return node
return go(root)