C++ Linked List Exercises: Insert new node at the middle of a Doubly Linked List

C++ Linked List: Exercise-21 with Solution

Write a C++ program to insert a new node at the middle of a given Doubly Linked List.

Test Data:
Doubly linked list is as follows:
---------------------------------
Traversal in Forward direction: Orange White Green Red
Traversal in Reverse direction: Red Green White Orange
Insert a new node at the middle position of the said Doubly linked list:
-----------------------------------------------------------------------
Traversal in Forward direction: Orange White Pink Green Red
Traversal in Reverse direction: Red Green Pink White Orange
Insert another new node at the middle position of the said Doubly linked list:
-----------------------------------------------------------------------------
Traversal in Forward direction: Orange White Pink Black Green Red
Traversal in Reverse direction: Red Green Black Pink White Orange

Sample Solution:

C++ Code:

#include <iostream> // Including input-output stream header file

using namespace std; // Using standard namespace

// A doubly linked list node
struct Node {
   string data; // Data field to store string data
   struct Node* next; // Pointer to the next node
   struct Node* prev; // Pointer to the previous node
   Node *link; // Additional pointer, not used in the code
};

// Function to append data at the front of the doubly linked list
void append_data(Node** head, string new_data)
{
   // Create a new node and allocate memory.
   struct Node* newNode = new Node;

   // Assign data to new node
   newNode->data = new_data;

   // A new node has been added with the name head and the previous node 
   // set to null, since it is being added at the front.
   newNode->next = (*head);
   newNode->prev = NULL;

   // Previous head is the new node
   if ((*head) != NULL)
   (*head)->prev = newNode;

   // Head points to the new node
   (*head) = newNode;
}

// Function to insert a node at the middle position of the doubly linked list
void insertAtMid(Node** head, string data){
    Node* new_Node = new Node(); // Creating a new node
    new_Node->data = data; // Assigning data to the new node

    if(*head == NULL){
        return; // If the linked list is empty, return
    }

    Node* ptr = *head;
    int size = 0;

    // Calculate length of the linked list, i.e., the number of nodes
    while (ptr != NULL) {
        size++;
        ptr = ptr->next;
    }

    // Find middle position
    int mid_pos = (size % 2 == 0) ? (size/2) : (size+1)/2;
    if(mid_pos == size){
        new_Node->next = NULL;
        new_Node->prev = *head;
        (*head)->next = new_Node;
        size++;
        return;
    }

    Node* temp = *head;
    while(--mid_pos){
        temp = temp->next;
    }

    (temp->next)->prev = new_Node;
    new_Node->prev = temp;
    new_Node->next = temp->next;
    temp->next = new_Node;
    size++;
}

// Following function displays contents of the doubly linked list
void displayDlList(Node* head)
{
    Node* last_node;
    cout << "\nTraversal in Forward direction: ";
    while (head != NULL) {
        cout << " " << head->data << " "; // Displaying data in forward direction
        last_node = head;
        head = head->next;
    }
    cout << "\nTraversal in Reverse direction: ";
    while (last_node != NULL) {
        cout << " " << last_node->data << " "; // Displaying data in reverse direction
        last_node = last_node->prev;
    }
}

// Main program
int main() {
   /* Start with the empty list */
   struct Node* head = NULL; // Initializing the head of the linked list as NULL
   append_data(&head, "Red"); // Appending "Red" at the front of the list
   append_data(&head, "Green"); // Appending "Green" at the front of the list
   append_data(&head, "White"); // Appending "White" at the front of the list
   append_data(&head, "Orange"); // Appending "Orange" at the front of the list  
   cout<<"Doubly linked list is as follows: ";
   cout<<"\n---------------------------------";
   displayDlList(head); // Displaying the doubly linked list
   cout<<"\n\nInsert a new node at the middle position of the said Doubly linked list:";
   cout<<"\n-----------------------------------------------------------------------";
   insertAtMid(&head, "Pink"); // Inserting "Pink" node at the middle of the list
   displayDlList(head); // Displaying the updated doubly linked list
   cout<<"\n\nInsert another new node at the middle position of the said Doubly linked list:";
   cout<<"\n-----------------------------------------------------------------------------";
   insertAtMid(&head, "Black"); // Inserting "Black" node at the middle of the list
   displayDlList(head); // Displaying the updated doubly linked list
   cout<<endl;
   return 0; // Returning from the main function
}

Sample Output:

Doubly linked list is as follows:
---------------------------------
Traversal in Forward direction:  Orange  White  Green  Red
Traversal in Reverse direction:  Red  Green  White  Orange

Insert a new node at the middle position of the said Doubly linked list:
-----------------------------------------------------------------------
Traversal in Forward direction:  Orange  White  Pink  Green  Red
Traversal in Reverse direction:  Red  Green  Pink  White  Orange

Insert another new node at the middle position of the said Doubly linked list:
-----------------------------------------------------------------------------
Traversal in Forward direction:  Orange  White  Pink  Black  Green  Red
Traversal in Reverse direction:  Red  Green  Black  Pink  White  Orange

Flowchart:

CPP Code Editor:

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Previous C++ Exercise: Middle element of a Doubly Linked List.