struct TreeNode* getSuccessor(struct TreeNode* node) {
    struct TreeNode* succ = node->right;
    while (succ->left != NULL && succ->left != node) {
        succ = succ->left;
    }
    return succ;
}

struct TreeNode* convertBST(struct TreeNode* root) {
    int sum = 0;
    struct TreeNode* node = root;

    while (node != NULL) {
        if (node->right == NULL) {
            sum += node->val;
            node->val = sum;
            node = node->left;
        } else {
            struct TreeNode* succ = getSuccessor(node);
            if (succ->left == NULL) {
                succ->left = node;
                node = node->right;
            } else {
                succ->left = NULL;
                sum += node->val;
                node->val = sum;
                node = node->left;
            }
        }
    }

    return root;
}