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:

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/

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;
    }
}