Doubly linked list

doublyLinkedList/DLL_question.java

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+//Given a doubly linked list. Your task is to reverse the doubly linked list and return its head.
+
+//solution code 
+
+class DLLNode {
+    int data;
+    DLLNode next;
+    DLLNode prev;
+
+    DLLNode(int val) {
+        data = val;
+        next = null;
+        prev = null;
+    }
+}
+
+class Solution {
+    // Function to reverse a doubly linked list
+    public DLLNode reverseDLL(DLLNode head) {
+        if (head == null) {
+            return null; // If the list is empty, return null
+        }
+
+        DLLNode curr = head;
+        DLLNode temp = null;
+
+        // Traverse the list and swap the next and prev pointers
+        while (curr != null) {
+            // Swap the next and prev pointers
+            temp = curr.prev;
+            curr.prev = curr.next;
+            curr.next = temp;
+
+            // Move to the next node (which is prev now due to the swap)
+            curr = curr.prev;
+        }
+
+        // After the loop, temp will be pointing to the new head of the reversed list
+        if (temp != null) {
+            head = temp.prev; // Set the new head
+        }
+
+        return head; // Return the new head of the reversed list
+    }
+}

doublyLinkedList/doublyLinkedList.java

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+// Node class representing each element of the doubly linked list
+class Node {
+    int data;
+    Node prev, next;
+
+    // Constructor to create a new node with a given data
+    public Node(int data) {
+        this.data = data;
+        prev = next = null;
+    }
+}
+
+// DoublyLinkedList class for basic operations
+public class DoublyLinkedList {
+    Node head;
+
+    // Constructor to initialize an empty doubly linked list
+    public DoublyLinkedList() {
+        head = null;
+    }
+
+    // Insert a new node at the end of the doubly linked list
+    public void insert(int data) {
+        Node newNode = new Node(data);
+
+        // If the list is empty, make the new node the head
+        if (head == null) {
+            head = newNode;
+            return;
+        }
+
+        // Otherwise, traverse to the end of the list
+        Node last = head;
+        while (last.next != null) {
+            last = last.next;
+        }
+
+        // Update the next and prev pointers to insert the new node
+        last.next = newNode;
+        newNode.prev = last;
+    }
+
+    // Delete a node with the given data from the doubly linked list
+    public void delete(int data) {
+        if (head == null) return; // List is empty, nothing to delete
+
+        Node current = head;
+
+        // Traverse the list to find the node to be deleted
+        while (current != null && current.data != data) {
+            current = current.next;
+        }
+
+        // Node not found
+        if (current == null) return;
+
+        // If the node to be deleted is the head
+        if (current == head) {
+            head = current.next;
+        }
+
+        // Update pointers to remove the node
+        if (current.next != null) {
+            current.next.prev = current.prev;
+        }
+
+        if (current.prev != null) {
+            current.prev.next = current.next;
+        }
+    }
+
+    // Traverse the list from the head to the end
+    public void traverseForward() {
+        Node current = head;
+        System.out.print("Doubly Linked List (forward): ");
+        while (current != null) {
+            System.out.print(current.data + " ");
+            current = current.next;
+        }
+        System.out.println();
+    }
+
+    // Traverse the list from the end to the head (reverse)
+    public void traverseBackward() {
+        if (head == null) return; // Empty list
+
+        // Traverse to the last node
+        Node last = head;
+        while (last.next != null) {
+            last = last.next;
+        }
+
+        // Traverse backward using the prev pointers
+        System.out.print("Doubly Linked List (backward): ");
+        while (last != null) {
+            System.out.print(last.data + " ");
+            last = last.prev;
+        }
+        System.out.println();
+    }
+
+    // Search for a node with the given data
+    public boolean search(int data) {
+        Node current = head;
+
+        // Traverse the list to find the data
+        while (current != null) {
+            if (current.data == data) return true;
+            current = current.next;
+        }
+
+        return false;
+    }
+
+    // Driver method for testing the doubly linked list implementation
+    public static void main(String[] args) {
+        DoublyLinkedList dll = new DoublyLinkedList();
+
+        // Inserting nodes into the doubly linked list
+        dll.insert(10);
+        dll.insert(20);
+        dll.insert(30);
+        dll.insert(40);
+        dll.insert(50);
+
+        // Traversing forward
+        System.out.println("Traversing forward:");
+        dll.traverseForward(); // Expected output: 10 20 30 40 50
+
+        // Traversing backward
+        System.out.println("\nTraversing backward:");
+        dll.traverseBackward(); // Expected output: 50 40 30 20 10
+
+        // Deleting a node
+        System.out.println("\nDeleting node 30:");
+        dll.delete(30);
+        dll.traverseForward(); // Expected output (without 30): 10 20 40 50
+
+        // Searching for elements
+        System.out.println("\nSearching for 40: " + dll.search(40)); // Output: true
+        System.out.println("Searching for 60: " + dll.search(60)); // Output: false
+    }
+}

doublyLinkedList/doublyLinkedList.txt

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+Doubly Linked List Documentation
+A Doubly Linked List (DLL) is a linear data structure where each node contains three fields: data, 
+a reference to the next node, and a reference to the previous node. Unlike a singly linked list,
+ a DLL allows traversal in both directions—forward and backward.
+
+Two Components:
+
+Node:
+1. Contains data.
+2. Contains a reference to the next node (next) and the previous node (prev).
+
+Head Node:
+1. The first node in the list.
+2. If the list is empty, the head is null.
+
+Operations:
+1. Insertion:
+New nodes can be inserted at the end of the list.
+Traversal happens from the head node to the end, where the new node is appended.
+
+2. Deletion:
+Deletes a node with a specific value.
+Traverses the list to find the node, and updates the next and prev pointers to remove the node.
+
+3. Traversal:
+Forward Traversal: Starts from the head and moves toward the end using the next pointer.
+Backward Traversal: Starts from the last node and moves backward using the prev pointer.
+
+4. Search:
+Searches for a node with a given data value.
+Traverses the list to find the node and returns true if found, otherwise false.
+
+
+Time Complexity:
+
+Insertion:
+O(n) in the worst case, as you may need to traverse the entire list to insert at the end.
+
+Deletion:
+O(n) to locate the node to delete (as traversal is required).
+
+Traversal:
+O(n), where n is the number of nodes in the list.
+
+Search:
+O(n), since you may need to traverse the entire list to find a specific node.