C++ Linked List Exercises: Middle element of a Doubly Linked List

C++ Linked List: Exercise-20 with Solution

Write a C++ program to find the middle element 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
The middle element is: Green
Insert a new node at the end of a Doubly Linked List:
-------------------------------------------------------------
Traversal in Forward direction: Pink Orange White Green Red
Traversal in Reverse direction: Red Green White Orange Pink
The middle element is: White

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 the 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 find and display the middle element of the doubly linked list
void Middle_element(Node** head)  
{  
    struct Node *slowptr = *head;  
    struct Node *fastptr = *head;  

    if (head!=NULL)  
    {  
        while (fastptr != NULL && fastptr->next != NULL)  
        {  
            fastptr = fastptr->next->next;  
            slowptr = slowptr->next;  
        }  
        cout<<"\nThe middle element is: "<<slowptr->data;  
    }  
}

// 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
   Middle_element(&head); // Finding and displaying the middle element
   cout<<"\n\nInsert a new node:";
   cout<<"\n--------------------";
   append_data(&head, "Pink"); // Inserting "Pink" node at the front of the list
   displayDlList(head); // Displaying the updated doubly linked list
   Middle_element(&head); // Finding and displaying the new middle element
   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
The middle element is: Green

Insert a new node:
--------------------
Traversal in Forward direction:  Pink  Orange  White  Green  Red
Traversal in Reverse direction:  Red  Green  White  Orange  Pink
The middle element is: White

Flowchart:

CPP Code Editor:

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Previous C++ Exercise: Insert a new node at the end of a Doubly Linked List.
Next C++ Exercise: Insert new node at the middle of a Doubly Linked List.