increment function

Hello,

Briefly to describe what i am trying to do: The user enters a number and the function increment  , increments the number by 1. The numbers a user enters are added to the front of the list, which makes the incrementation easier and plus some other operations. Current display function is for helping to see how incrementation work.
Can you please have a look at the increment function. I have put 3 options:
1. if number = null
2. if value in the first node is from 0 to 8
3. if value in the first node is 9

Please try to input

52, it will return 53
59 will return 60
but 58 ill return 60 - and that is the problem i am having.

if i put the if statement (if value in the first node is from 0 to 8) after statement (if value in the first node is 9), then when user inputs 59, it gives them an error. Cant find why it is occuring so ?
Can you pls advise where is the bug?

#include<iostream.h>
#include<string>
typedef int ListItemType;

class Newstring

{
private:

    struct digit // a node on the List
    {
      ListItemType item; // data item on the list
      digit *next; // pointer to the next node
      };

     digit *number;
     digit *head;    // Points to front end of list.
     digit *tail;    // Points to a node one past the end of list.
   
public:

Newstring():head(NULL),tail(NULL){}
     
void addtofront ( ListItemType );
void increment ();
void display ();

~Newstring();
};


void Newstring::addtofront (ListItemType n)
{    
     digit* pNewDigit = new digit();
     pNewDigit->item = n;
     pNewDigit->next = NULL;

     if (head == NULL)  
       {
          head = pNewDigit;

     }
      
       else
       {
          digit* pCurrentHead = head;
          head = pNewDigit;
          pNewDigit->next = pCurrentHead;
     }

}

void Newstring::increment ()

{
      digit *current=head;

if (head==NULL)
      {
            digit *current=head;
                  current->item =1;
      }

if(current->item<9 && current->item >=0)
{
                  current->item=current->item++;
}


if(current->item==9 && current->next!=NULL){

for( current=head; current!=NULL;current=current->next) {

if(current->item==9)
      current->item=0;

else
current->item++;

}
}


}


Newstring::~Newstring()
{
     digit* pTemp = head;

     while(pTemp)
     {
          head = head->next;
          delete pTemp;
          pTemp = head;
     }
     head = NULL;    
}

void Newstring::display()
{
     digit* pFirst = head;

     while (pFirst) {
          cout << pFirst->item << endl;
          pFirst = pFirst->next;
     }
}



int main ()
{

Newstring number1;

ListItemType n;

cout<<"Insert number: ";
cin>>n;
while (n!=-1)
{
cout<<"Insert number: ";
number1.addtofront(n);
cin>>n;
}
number1.display ();
number1.increment();
cout<<"After incrementation: "<<endl;
number1.display ();

      return 0;
}