# 538. Convert BST to Greater 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:**

![](https://assets.leetcode.com/uploads/2019/05/02/tree.png)

```
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 `[0, 104]`.
* `-104 <= Node.val <= 104`
* All the values in the tree are **unique**.
* `root` is guaranteed to be a valid binary search tree.

&#x20;

**Note:** This question is the same as 1038: <https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/>

```python
# 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 __init__(self):
        self.total = 0
        
    def convertBST(self, root: Optional[TreeNode], count: int = 0) -> Optional[TreeNode]:
        if root is not None:
            self.convertBST(root.right)
            self.total += root.val
            root.val = self.total
            self.convertBST(root.left)
        return root
            
        
            
```