Added Circular Doubly Linked List in the datastructures section (#6565)

src/main/java/com/thealgorithms/datastructures/lists/CircularDoublyLinkedList.java

@@ -0,0 +1,118 @@
+package com.thealgorithms.datastructures.lists;
+
+/**
+ * This class is a circular doubly linked list implementation. In a circular
+ * doubly linked list,
+ * the last node points back to the first node and the first node points back to
+ * the last node,
+ * creating a circular chain in both directions.
+ *
+ * This implementation includes basic operations such as appending elements to
+ * the end,
+ * removing elements from a specified position, and converting the list to a
+ * string representation.
+ *
+ * @param <E> the type of elements held in this list
+ */
+public class CircularDoublyLinkedList<E> {
+    static final class Node<E> {
+        Node<E> next;
+        Node<E> prev;
+        E value;
+
+        private Node(E value, Node<E> next, Node<E> prev) {
+            this.value = value;
+            this.next = next;
+            this.prev = prev;
+        }
+    }
+
+    private int size;
+    Node<E> head = null;
+
+    /**
+     * Initializes a new circular doubly linked list. A dummy head node is used for
+     * simplicity,
+     * pointing initially to itself to ensure the list is never empty.
+     */
+    public CircularDoublyLinkedList() {
+        head = new Node<>(null, null, null);
+        head.next = head;
+        head.prev = head;
+        size = 0;
+    }
+
+    /**
+     * Returns the current size of the list.
+     *
+     * @return the number of elements in the list
+     */
+    public int getSize() {
+        return size;
+    }
+
+    /**
+     * Appends a new element to the end of the list. Throws a NullPointerException
+     * if
+     * a null value is provided.
+     *
+     * @param value the value to append to the list
+     * @throws NullPointerException if the value is null
+     */
+    public void append(E value) {
+        if (value == null) {
+            throw new NullPointerException("Cannot add null element to the list");
+        }
+        Node<E> newNode = new Node<>(value, head, head.prev);
+        head.prev.next = newNode;
+        head.prev = newNode;
+        size++;
+    }
+
+    /**
+     * Returns a string representation of the list in the format "[ element1,
+     * element2, ... ]".
+     * An empty list is represented as "[]".
+     *
+     * @return the string representation of the list
+     */
+    public String toString() {
+        if (size == 0) {
+            return "[]";
+        }
+        StringBuilder sb = new StringBuilder("[ ");
+        Node<E> current = head.next;
+        while (current != head) {
+            sb.append(current.value);
+            if (current.next != head) {
+                sb.append(", ");
+            }
+            current = current.next;
+        }
+        sb.append(" ]");
+        return sb.toString();
+    }
+
+    /**
+     * Removes and returns the element at the specified position in the list.
+     * Throws an IndexOutOfBoundsException if the position is invalid.
+     *
+     * @param pos the position of the element to remove
+     * @return the value of the removed element - pop operation
+     * @throws IndexOutOfBoundsException if the position is out of range
+     */
+    public E remove(int pos) {
+        if (pos >= size || pos < 0) {
+            throw new IndexOutOfBoundsException("Position out of bounds");
+        }
+        Node<E> current = head.next;
+        for (int i = 0; i < pos; i++) {
+            current = current.next;
+        }
+        current.prev.next = current.next;
+        current.next.prev = current.prev;
+        E removedValue = current.value;
+        size--;
+        return removedValue;
+    }
+}