Add O(1) queue data structure

Author: cszach

src/main/lists/Queue.java

@@ -0,0 +1,70 @@
+/**
+ * An implementation of the queue data structure based on linked list.
+ *
+ * <p>
+ * This implementation is basically a linked list with a tail, where the head
+ * of the list is the front of the queue and the tail is the rear. Both the head
+ * and the tail are empty nodes themselves. The rear (the last enqueued element)
+ * points to the tail and the tail points back to the rear, forming a cycle.
+ * This helps achieve both enqueueing and dequeueing in constant time.
+ *
+ * <p>
+ * Operations with their complexities are:
+ *   <ul>
+ *     <li><code>enqueue</code>: O(1)</li>
+ *     <li><code>dequeue</code>: O(1)</li>
+ *     <li><code>peek</code>: O(1)</li>
+ *   </ul>
+ */
+public class Queue<T> {
+  private LinkedListNode<T> head;
+  private LinkedListNode<T> tail;
+  private int length;
+
+  public Queue() {
+    this.head = new LinkedListNode<T>(null, null);
+    this.tail = new LinkedListNode<T>(null, this.head);
+    this.head.next = this.tail;
+    this.length = 0;
+  }
+
+  public Queue(T[] data) {
+    this.head = new LinkedListNode<T>(null, null);
+    this.tail = new LinkedListNode<T>(null, this.head);
+    this.head.next = this.tail;
+    this.length = 0;
+
+    for (T d: data)
+      this.enqueue(d);
+  }
+
+  public int getLength() {
+    return this.length;
+  }
+
+  public boolean isEmpty() {
+    return this.length == 0;
+  }
+
+  public Queue<T> enqueue(T data) {
+    this.tail.next.next = new LinkedListNode<T>(data, this.tail);
+    this.tail.next = this.tail.next.next;
+
+    this.length++;
+    return this;
+  }
+
+  public Queue<T> dequeue() {
+    if (this.length == 0) return this;
+
+    this.head.next = this.head.next.next;
+
+    this.length--;
+    if (this.length == 0) this.tail.next = this.head;
+    return this;
+  }
+
+  public T peek() {
+    return this.head.next.data;
+  }
+}