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/**
 * 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;

    TreeNode* convertBST(TreeNode* root) {
        if (root != nullptr) {
            convertBST(root->right);
            sum += root->val;
            root->val = sum;
            convertBST(root->left);
        }

        return root;
    }
};