/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
const convertBST = function (root) {
  const value = { sum: 0 }
  inorderTraversal(root, value);
  return root;
};
const inorderTraversal = function (root, value) {
  if (root === null) {
    return;
  }
  inorderTraversal(root.right);
  sum += root.val;
  root.val = sum;
  inorderTraversal(root.left);
};