### LeetCode Challenge #1038. Binary Search Tree to Greater Sum Tree

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.
##### Video Solution
###### Java Solution
```				```
class Solution {
int sum = 0 ;
public TreeNode bstToGst(TreeNode root) {
helper(root);
return root ;
}

public void helper(TreeNode root){
if(root == null){
return ;
}
helper(root.right);
sum = sum + root.val;
root.val = sum ;
helper(root.left);
}
}
```
```