Create a doubly linked list from a singly

[MisterTams]

How can I create a doubly linked list?
I overall understand the concept of the singly linked list but the doubly is giving me a hard time.

I cannot for the life of me figure how to add a node at the beginning of the list or at the middle. I have looked at examples online but they don't really seem to help. I think I have it figured out for adding at the end of the list. I also am having a hard time linking the previous nodes to one another to create a 'doubly'. I say this because obviously I am doing something wrong because I can display my list forwards correctly but when I try to display it in reverse order it will only show my last element in the list.

Also, I have to make the node in main(). So no, I cannot use the OOP approach. I also cannot use the STL list.

Here is my code so far...

#include <iostream>
using namespace std;

int main()
{
	struct ListNode
	{
		int value;
		ListNode *next;
		ListNode *prev;
	};

	ListNode *head = nullptr;
	ListNode *tail = nullptr;
	ListNode *newNode = nullptr;
	ListNode *nodePtr = nullptr;
	ListNode *prevPtr = nullptr;
	ListNode *nextNode = nullptr;
	ListNode *temp = nullptr;

	char ch = 'q';
	int val;

	do {
		cout << "a-add element to the list\n";
		cout << "d-delete element in the list\n";
		cout << "p-print this list\n";
		cout << "q-quit\n";
		cin >> ch;

		switch (ch) {
		case 'a':
			cout << "1-add element at beginning, 2-add at end, 3-add after an item\n";
			cout << "choice: ";
			cin >> ch;
			cout << "Enter the value to be added: ";
			cin >> val;

			newNode = new ListNode;
			newNode->value = val;
			newNode->next = nullptr;
			newNode->prev = nullptr;
			
			if (ch == '1') //add element at beginning
			{
				if (head == nullptr)
				{
					head = newNode;
					tail = newNode;
				}
				else if (head != nullptr)
				{
					nodePtr = head;
					temp = head;
					newNode->prev = head;
					head = newNode;
					nodePtr->next = temp;

					while (nodePtr->next != nullptr)
					{
						nodePtr->prev = nodePtr;
						nodePtr = nodePtr->next;
					}


				}

			}
			else if (ch == '2') //add at end
			{
				if (head == nullptr)
				{
					head = newNode;
					tail = newNode;
				}
				else
				{
					nodePtr = head;
					
					
					while (nodePtr->next != nullptr)
					{
						nodePtr->prev = nodePtr;
						nodePtr = nodePtr->next;
					}

					nodePtr->next = newNode;
					tail->next = newNode;
					tail = newNode;
				}

				cout << "\nhead = " << head->value;
				cout << "\ntail = " << tail->value << endl;
			}
			else if (ch == '3') //add after an item
			{


			}
			


			break;
		case 'd':
			cout << "deleting...\n";
			cout << "enter a value to be deleted ";
			cin >> val;

			if (head != nullptr)
			{
				if (head->value == val)
				{
					nodePtr = head->next;
					delete head;
					head = nodePtr;
				}
				else
				{
					nodePtr = head;

					while (nodePtr != nullptr && nodePtr->value != val)
					{
						prevPtr = nodePtr;
						nodePtr = nodePtr->next;
					}
					if (nodePtr != nullptr)
					{
						prevPtr->next = nodePtr->next;
						delete nodePtr;
					}
				}

			}
			else
				cout << "list is empty" << endl;
			break;
		case 'p':
			cout << "1-print forward, 2-print in reverse\n";
			cout << "choice: ";
			cin >> ch;

			if (ch == '1')
			{
				nodePtr = head;
				while (nodePtr != nullptr)
				{
					cout << "elem: " << nodePtr->value << endl;
					nodePtr = nodePtr->next;
				}
			}
			
			else if (ch == '2')
			{
				nodePtr = tail;
				while (nodePtr != nullptr)
				{
					cout << "elem: " << nodePtr->value << endl;
					nodePtr = nodePtr->prev;
				}
			}
			break;
		case 'q':
			cout << "quitting...\n";
			nodePtr = head;

			while (nodePtr != nullptr)
			{
				nextNode = nodePtr->next;
				cout << " deleting ... " << nodePtr->value << endl;
				delete nodePtr;
				nodePtr = nextNode;

			}
			break;
		default:
			cout << "invalid character -- please try again\n";
			break;
		}

	} while (ch != 'q');

	cout << endl;
	system("pause");
	return 0;
}

[Duthomhas]

When dealing with linked lists, it is always a good idea to get out a piece of paper and a pencil and start drawing.

A singly linked list:

              ┌───┐   ┌───┐   ┌───┐   ┌───┐
      head ──►│ 0 ├──►│ 1 ├──►│ 2 ├──►│ 3 ├──► nullptr
              └───┘   └───┘   └───┘   └───┘

A doubly linked list also has pointers going back the the previous node:

              ┌───┐   ┌───┐   ┌───┐   ┌───┐
      head ──►│ 0 ├──►│ 1 ├──►│ 2 ├──►│ 3 ├──► nullptr
   nullptr ◄──┤   │◄──┤   │◄──┤   │◄──┤   │◄── tail
              └───┘   └───┘   └───┘   └───┘

To add or remove nodes, work out how to adjust the arrows (pointers).

Hope this helps.