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# Binary Search Tree to Greater Sum Tree

Given the root of a binary **search** tree with distinct values, modify it so that every `node` has a new value equal to the sum of the values of the original tree that are greater than or equal to `node.val`.

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: [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]
```

**Constraints:**

1. The number of nodes in the tree is between `1` and `100`.
2. Each node will have value between `0` and `100`.
3. The given tree is a binary search tree.

**Note:** This question is the same as 538: <https://leetcode.com/problems/convert-bst-to-greater-tree/>

```java
class Solution {
    public TreeNode bstToGst(TreeNode root) {
        helper(root, 0);
        return root;
    }

    public int helper(TreeNode root, int parentSum) {
        if (root == null)
            return 0;
        int rightSum = helper(root.right, parentSum);
        int leftSum = helper(root.left, root.val + rightSum + parentSum);
        int treeSum = root.val + leftSum + rightSum;
        root.val += rightSum + parentSum;
        return treeSum;
    }
}
```