'''/**

• Definition for a binary tree node.
• struct TreeNode {

• int val;

• TreeNode *left;

• TreeNode *right;

• TreeNode() : val(0), left(nullptr), right(nullptr) {}

• TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}

• TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
• };
  /
  class Solution {
  public:
  int sum = 0;
  TreeNode convertBST(TreeNode* root) {
  if(root == NULL){
  return NULL;
  }
  convertBST(root->right);
  root->val = root->val + sum;
  sum = root->val;
  convertBST(root->left);
  return root;
  }
  };'''