Create DequeUsingArrayOrLinkedList.java

Author: Rahilralu

src/test/java/com/thealgorithms/datastructures/queues/DequeUsingArrayOrLinkedList.java

@@ -0,0 +1,128 @@
+package com.thealgorithms.queues;
+
+import java.util.NoSuchElementException;
+
+/**
+ * Implementation of a Double-Ended Queue (Deque) using a doubly linked list.
+ * <p>
+ * A deque allows insertion and deletion from both the front and the rear.
+ * It follows the FIFO principle at both ends.
+ * </p>
+ * 
+ * Example:
+ * DequeUsingArrayOrLinkedList<Integer> deque = new DequeUsingArrayOrLinkedList<>();
+ * deque.addFront(1);
+ * deque.addRear(2);
+ * deque.removeFront(); // returns 1
+ * deque.removeRear();  // returns 2
+ * 
+ * @param <T> the type of elements stored in the deque
+ */
+public final class DequeUsingArrayOrLinkedList<T> {
+
+    private Node<T> front;
+    private Node<T> rear;
+    private int size;
+
+    /** Node class for doubly linked list */
+    private static class Node<T> {
+        T data;
+        Node<T> prev;
+        Node<T> next;
+
+        Node(T data) {
+            this.data = data;
+        }
+    }
+
+    /** Constructor to initialize an empty deque */
+    public DequeUsingArrayOrLinkedList() {
+        front = null;
+        rear = null;
+        size = 0;
+    }
+
+    /** Add an element at the front of the deque */
+    public void addFront(T value) {
+        Node<T> newNode = new Node<>(value);
+        if (isEmpty()) {
+            front = rear = newNode;
+        } else {
+            newNode.next = front;
+            front.prev = newNode;
+            front = newNode;
+        }
+        size++;
+    }
+
+    /** Add an element at the rear of the deque */
+    public void addRear(T value) {
+        Node<T> newNode = new Node<>(value);
+        if (isEmpty()) {
+            front = rear = newNode;
+        } else {
+            rear.next = newNode;
+            newNode.prev = rear;
+            rear = newNode;
+        }
+        size++;
+    }
+
+    /** Remove and return the element from the front of the deque */
+    public T removeFront() {
+        if (isEmpty()) {
+            throw new NoSuchElementException("Deque is empty");
+        }
+        T value = front.data;
+        front = front.next;
+        if (front != null) {
+            front.prev = null;
+        } else {
+            rear = null; // deque became empty
+        }
+        size--;
+        return value;
+    }
+
+    /** Remove and return the element from the rear of the deque */
+    public T removeRear() {
+        if (isEmpty()) {
+            throw new NoSuchElementException("Deque is empty");
+        }
+        T value = rear.data;
+        rear = rear.prev;
+        if (rear != null) {
+            rear.next = null;
+        } else {
+            front = null; // deque became empty
+        }
+        size--;
+        return value;
+    }
+
+    /** Peek the front element without removing it */
+    public T peekFront() {
+        if (isEmpty()) {
+            throw new NoSuchElementException("Deque is empty");
+        }
+        return front.data;
+    }
+
+    /** Peek the rear element without removing it */
+    public T peekRear() {
+        if (isEmpty()) {
+            throw new NoSuchElementException("Deque is empty");
+        }
+        return rear.data;
+    }
+
+    /** Check if the deque is empty */
+    public boolean isEmpty() {
+        return size == 0;
+    }
+
+    /** Return the number of elements in the deque */
+    public int size() {
+        return size;
+    }
+}