what is wrong with the else statement , i am converting string to int to string to linked list

Question

If we have a linked list and pointer to its head. Linked list represent a number. Then add 1 to the number and output the new linked list. For ex 2->6->9 ie 269 should output 270 ie 2->7>0.

My code is running fine when there is no carry, but not otherwise.

I am first taking the number in a string, converting it to a number and converting it back to a string creating the new linked list of the resultant after adding 1 to it.

Node* addOne(Node *head) 
    {
        Node*ptr=head;
        while(ptr->next!=NULL){
            ptr=ptr->next;
        }
        if(ptr->data<9){
        ptr->data=ptr->data+1;
        }
        else{
            ptr=head;
            string s="";
            while(ptr!=NULL){
                int data=ptr->data;
                s=s+to_string(data);
                ptr=ptr->next;
            }
            int x;
            x=stoi(s);
            x++;
            s=to_string(x);
            int n=s.length();
            Node*head2=(Node*)malloc(sizeof(Node*));
            Node*ptr2=head2;
            int i=0;
            ptr2->data=s[i];
            ptr2->next=NULL;
            for(i=1;i<n;i++){
                Node*temp=(Node*)malloc(sizeof(Node*));
                temp->data=s[i];
                ptr2->next=temp;
                temp->next=NULL;
                ptr2=ptr2->next;
            }
            return head2;
        }
        return head;
        
    }

Input :

369

Ouput:

515548

Expected Output:

370