#include <iostream.h>

class ListElem
{
public:
   int      value;      // Integer
   ListElem *next;      // Pointer used to make link    
};


void Print(ListElem *head)
{
   ListElem *p;

   p = head;             // Very important: start at first element

   cout << "List ==> ";
   while ( p != NULL )
   {
      cout << p->value << "  ";  // Access value stored in list element  
      p = p->next;               // Visit next element in list
   }
   cout << "\n\n";
}

/* -----------------------------------------------------------
   DeleteAtTail: Delete elem at TAIL of linked list

   Question: what would happen if we pass "head" by value ?
   ----------------------------------------------------------- */
void DeleteAtTail(ListElem * & head)
{
    ListElem *p2;
    ListElem *p1;

    if ( head == NULL ) // Delete only from non-empty list
    {
       return;
    }
    else
    {
       if ( head->next == NULL )
       {  // Handle special case: List has 1 element

	  delete head;	    // Deallocate space

	  head = NULL;
       }
       else
       {  // The general case

	  // Find the last-but-one "p1" and last "p2" elements

	  p1 = head;
	  p2 = head->next;

	  while ( p2->next != NULL )
	  {
	     p1 = p2;
	     p2 = p2->next;
	  }

	  // Delete element following the "p1" element    
	  p1->next = NULL;
	  delete p2;	    // Deallocate space
       }

    }
}


/* -----------------------------------------------------------
   InsertAtHead: insert elem at start of linked list

   Question: what would happen if we pass "head" by value ?
   ----------------------------------------------------------- */
void InsertAtHead(ListElem * & head, ListElem * elem)
{
   elem->next = head;    // Make new list element points   
                         // to the start of "old list"   

   head = elem;          // Make "head" points to 
                         // the **new** starting object
}




int main(int argc, char *argv[])
{
   ListElem *head, *p;
   extern void Print(ListElem *);
   extern void InsertAtHead(ListElem * &, ListElem *);
   extern void DeleteAtTail(ListElem * &);

   head = NULL;
   Print(head);

   p = new ListElem;
   p->value = 1500;
   InsertAtHead(head, p);
   Print(head);
   
   p = new ListElem;
   p->value = 2500;
   InsertAtHead(head, p);
   Print(head);
   
   p = new ListElem;
   p->value = 3500;
   InsertAtHead(head, p);
   Print(head);

   // ========================================== Removing...

   DeleteAtTail(head);
   Print(head);

   DeleteAtTail(head);
   Print(head);

   DeleteAtTail(head);
   Print(head);

   DeleteAtTail(head);
   Print(head);
}