remember to update the value of predecessor of root before recursively goes to left child

```
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode* convertBST(TreeNode* root) {
if(!root) return root;
convertBST(root -> right);
auto node = root -> right;
while(node && node -> left) node = node -> left;
if(node) root -> val += node -> val;
node = root -> left;
while(node && node -> right) node = node -> right;
if(node) node -> val += root -> val;
convertBST(root -> left);
return root;
}
};
```