How can I convert this single linked list into a double linked list that has the

Question

How can I convert this single linked list into a double linked list that has the function; delete, reverse print, and insert

any help is appreciated thanks in advance

using namespace std;
#include<iostream>
#include<sstream>
#include<string>
#include<cstdlib>

class Stock
{
   public:
   int info;
   Stock *link;
   Stock *back;
  
  
   string stockSymbol;
   double cost;
   int shares;
};

Stock* generateStocks();
void printStocks(Stock *head);
Stock* MiddleList(Stock *head);
Stock* SplitStocks(Stock *head);

//generate the linked list Stocks
Stock* generateStocks()
{
   Stock *front, *end, *newNode;
   front = NULL;
   end = NULL;

   for(int i = 1; i < 21; i++)
   {
       stringstream s;
       s << i;
       newNode = new Stock;
       newNode ->cost = rand() % 100;
       newNode ->shares = rand() % 100;
       newNode ->stockSymbol = s.str();
       newNode ->link = NULL;

       if(front == NULL)
       {
           front = newNode;
           end = newNode;
       }

       else
       {
           end ->link = newNode;
           end = newNode;
       }
   }
   newNode = front;
   return newNode;
}

//print the Stocks
void printStocks(Stock *head)
{
   Stock *newNode = head;
   while(newNode != NULL)
   {
       cout << "Stock Symbol:A" << newNode ->stockSymbol << " Cost:$" << newNode ->cost
       << " Shares:" << newNode ->shares << endl;
       newNode = newNode ->link;
   }
       cout << endl;

}

//find and print the element found in the middle of the stocks
Stock* MiddleList(Stock *head)
{
   Stock *slow = head, *fast = head;
   int pos = 1;
   int count = 0;
   while(fast ->link != NULL)
   {
       count++;
       if(slow == NULL)
       {
           exit(-1);
       }
       fast = fast->link;
       if(count % 2 == 0)
       {
           slow = slow->link;
           pos++;
       }
   }
   cout << "The middle element is: A" << slow ->stockSymbol << " Cost:$" << slow ->cost
   << " Shares:" << slow ->shares << endl;
   cout << "The position of the stock is: " << pos << endl << endl;

   return slow;
}

//split the stocks in half
Stock* SplitStocks(Stock *head)
{
   Stock *slow = head, *fast = head;
   int pos = 1;
   int count = 0;
   while(fast ->link != NULL)
   {
       count++;
       if(slow == NULL)
       {
           exit(-1);
       }
       fast = fast->link;
       if(count % 2 == 0)
       {
           slow = slow->link;
           pos++;
       }
   }
   Stock *head2 = slow ->link;
   slow ->link = NULL;

   return head2;
}

int main()
{
   cout << "Here are the 20 stocks! " << endl;
   Stock *head = generateStocks();
   printStocks(head);

   Stock *a = MiddleList(head);
   Stock *b = SplitStocks(head);

   cout << "Stocks found in the first half of the list " << endl;
   printStocks(head);
   cout << "Stocks found in the second half of the list " << endl;
   printStocks(b);

}

Explanation / Answer

In order to make a single linked list into a doubly one,you will need a pointer to point back to the previous node. So you will have a node that points to the next pointer and it will also point to the previous one.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct listNode
{
char * name;
struct listNode * nextPtr;
};
void print_function ( struct listNode *startPtr);
void insert_function(struct listNode **startPtr, char *data);
void delete_function(struct listNode **startPtr, char *value);
main()
{
char name[128];
struct listNode * startPtr = NULL;printf("enter the names you want to insert into a linked list ");
print_function(startPtr);
printf("enter name 1 ");
scanf("%s",name);
insert_function(&startPtr,name);
print_function(startPtr);
printf(" ");
printf("enter name 2 ");
scanf("%s", name);
insert_function(&startPtr,name);
print_function(startPtr);
printf(" ");
printf("enter name 3 ");
scanf("%s", name);
insert_function(&startPtr,name);
print_function(startPtr);
printf(" ");
printf("enter a name to delete ");
scanf("%s",name);
delete_function(&startPtr,name);   
print_function(startPtr);
printf(" ");
}
void insert_function(struct listNode **startPtr, char *value)
{
struct listNode *prevPtr = NULL;
struct listNode *currentPtr = NULL;
struct listNode *newPtr = NULL;
int string = strlen(value);
currentPtr = *startPtr;
newPtr = (struct listNode *)malloc(sizeof(struct listNode));
if (newPtr == NULL)
{
printf("can't allocate memory ");
return;
}
newPtr->name = (char *)malloc(string *sizeof(char));
strncpy(newPtr->name,value,string);
newPtr->nextPtr = NULL;
if (*startPtr == NULL)
{
*startPtr = newPtr;
return;
}
while (currentPtr != NULL && strncmp(newPtr->name,currentPtr->name,string) >0)
{
prevPtr = currentPtr;
currentPtr = currentPtr->nextPtr;
}
if (prevPtr == NULL)
{
newPtr->nextPtr = currentPtr;
*startPtr = newPtr;
return;
}
if (currentPtr == NULL)
{
prevPtr->nextPtr = newPtr;
return;
}
prevPtr->nextPtr = newPtr;
newPtr->nextPtr = currentPtr;
}
void delete_function(struct listNode **startPtr, char *value)
{
struct listNode *currentPtr =NULL;
struct listNode *prev =NULL;
currentPtr = *startPtr;
if(*startPtr == NULL)
{
printf("the list is empty, can't delete ");
return;
}
if((*startPtr)->name == value)
{
printf("list beginning ");
*startPtr= currentPtr->nextPtr;
free(currentPtr);
return;
}
while(currentPtr!=NULL && (strcmp(value,currentPtr->name) != 0))
{
prev = currentPtr;
currentPtr = currentPtr->nextPtr;
}
if(currentPtr == NULL)
{
printf("name not in list ");
return;
}
prev->nextPtr = currentPtr->nextPtr;
}
void print_function ( struct listNode *startPtr)
{
struct listNode *currentPtr = startPtr;
while(currentPtr!=NULL)
{
printf("|%2s| -->",currentPtr->name);
currentPtr = currentPtr->nextPtr;
}
printf(" ");
}

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