From 5aca2a65c8f9898d2de0f1832c16c35a66438fc3 Mon Sep 17 00:00:00 2001
From: SS56Bo <sohamsarkar345@gmail.com>
Date: Sat, 4 Oct 2025 11:55:57 +0530
Subject: [PATCH] Added Double Linked List implementation in Java

---
 DoublyLL.java | 189 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 189 insertions(+)
 create mode 100644 DoublyLL.java

diff --git a/DoublyLL.java b/DoublyLL.java
new file mode 100644
index 0000000..1899407
--- /dev/null
+++ b/DoublyLL.java
@@ -0,0 +1,189 @@
+class Node {
+    int data;
+    Node prev;
+    Node next;
+
+    Node(int x) {
+        data = x;
+        prev = next = null;
+    }
+}
+
+public class DoublyLL {
+
+    // 🧩 Insert at beginning
+    public Node insertAtBeginning(Node head, int x) {
+        Node temp = new Node(x);
+        temp.next = head;
+        if (head != null) {
+            head.prev = temp;
+        }
+        return temp;
+    }
+
+    // 🧩 Insert at end
+    public Node insertAtEnd(Node head, int x) {
+        Node temp = new Node(x);
+        if (head == null) return temp;
+
+        Node curr = head;
+        while (curr.next != null) {
+            curr = curr.next;
+        }
+        curr.next = temp;
+        temp.prev = curr;
+        return head;
+    }
+
+    // 🧩 Insert at specific position (1-based index)
+    public Node insertAtPosition(Node head, int pos, int x) {
+        if (pos <= 0) {
+            System.out.println("Invalid position!");
+            return head;
+        }
+
+        if (pos == 1) return insertAtBeginning(head, x);
+
+        Node temp = new Node(x);
+        Node curr = head;
+
+        for (int i = 1; i < pos - 1 && curr != null; i++) {
+            curr = curr.next;
+        }
+
+        if (curr == null) {
+            System.out.println("Position out of bounds!");
+            return head;
+        }
+
+        temp.next = curr.next;
+        if (curr.next != null) {
+            curr.next.prev = temp;
+        }
+        curr.next = temp;
+        temp.prev = curr;
+
+        return head;
+    }
+
+    // 🧩 Delete from beginning
+    public Node deleteAtBeginning(Node head) {
+        if (head == null) return null;
+        head = head.next;
+        if (head != null) {
+            head.prev = null;
+        }
+        return head;
+    }
+
+    // 🧩 Delete from end
+    public Node deleteAtEnd(Node head) {
+        if (head == null || head.next == null) return null;
+
+        Node curr = head;
+        while (curr.next != null) {
+            curr = curr.next;
+        }
+
+        curr.prev.next = null;
+        return head;
+    }
+
+    // 🧩 Delete at specific position (1-based index)
+    public Node deleteAtPosition(Node head, int pos) {
+        if (head == null || pos <= 0) return head;
+
+        if (pos == 1) return deleteAtBeginning(head);
+
+        Node curr = head;
+
+        for (int i = 1; i < pos && curr != null; i++) {
+            curr = curr.next;
+        }
+
+        if (curr == null) {
+            System.out.println("Position out of bounds!");
+            return head;
+        }
+
+        if (curr.next != null) {
+            curr.next.prev = curr.prev;
+        }
+        if (curr.prev != null) {
+            curr.prev.next = curr.next;
+        }
+
+        return head;
+    }
+
+    // 🧩 Reverse the doubly linked list
+    public Node reverse(Node head) {
+        if (head == null) return null;
+
+        Node curr = head, prevNode = null;
+        while (curr != null) {
+            prevNode = curr.prev;
+            curr.prev = curr.next;
+            curr.next = prevNode;
+            curr = curr.prev;
+        }
+
+        if (prevNode != null) {
+            head = prevNode.prev;
+        }
+        return head;
+    }
+
+    // 🧩 Print list forward
+    public void printList(Node head) {
+        Node curr = head;
+        System.out.print("Forward: ");
+        while (curr != null) {
+            System.out.print(curr.data + " ");
+            curr = curr.next;
+        }
+        System.out.println();
+    }
+
+    // 🧩 Print list backward
+    public void printReverse(Node head) {
+        if (head == null) return;
+        Node curr = head;
+        while (curr.next != null) {
+            curr = curr.next;
+        }
+        System.out.print("Backward: ");
+        while (curr != null) {
+            System.out.print(curr.data + " ");
+            curr = curr.prev;
+        }
+        System.out.println();
+    }
+
+    // 🧩 Example usage
+    public static void main(String[] args) {
+        DoublyLL dll = new DoublyLL();
+        Node head = null;
+
+        head = dll.insertAtEnd(head, 10);
+        head = dll.insertAtEnd(head, 20);
+        head = dll.insertAtEnd(head, 30);
+        head = dll.insertAtBeginning(head, 5);
+        head = dll.insertAtPosition(head, 3, 15);
+
+        dll.printList(head);
+        dll.printReverse(head);
+
+        head = dll.deleteAtBeginning(head);
+        head = dll.deleteAtEnd(head);
+        head = dll.deleteAtPosition(head, 2);
+
+        dll.printList(head);
+        dll.printReverse(head);
+
+        head = dll.reverse(head);
+        System.out.println("After reversal:");
+        dll.printList(head);
+        dll.printReverse(head);
+    }
+}