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LinkedList: Queue and Deque Operations

The LinkedList class in Java is a versatile data structure that can be used for various purposes. One of the important use cases of LinkedList is implementing queue and deque operations efficiently. In this article, we will explore how LinkedList can be used as a queue and deque and discuss the various operations associated with them.

Queue Operations

A queue follows the First-In-First-Out (FIFO) principle, where elements are added at the end and removed from the front. LinkedList provides all the necessary methods to implement a queue efficiently. Let's take a look at the major queue operations supported by LinkedList:

  1. Enqueue (add): The add method in LinkedList allows us to add an element to the end of the queue. It appends the element to the tail of the list, making it the last element.
LinkedList<String> queue = new LinkedList<>();
queue.add("element1");
queue.add("element2");
  1. Dequeue (remove): The remove method in LinkedList can be used to remove the element at the front of the queue. It retrieves and removes the first element of the list, resulting in the rest of the elements getting shifted up.
queue.remove(); // removes the first element
  1. Peek (retrieve): The peek method returns the element at the front of the queue without removing it. It is useful to check the element at the head before performing any further operations.
String head = queue.peek(); // retrieves the first element

Deque Operations

A deque (Double Ended Queue) allows adding and removing elements from both ends. It provides flexibility in implementing various data structures such as stacks, queues, or double-ended queues themselves. LinkedList supports deque operations efficiently. Here are the major deque operations available with LinkedList:

  1. Push (addFirst): The addFirst method adds an element to the beginning of the deque. It inserts the specified element at the front of the list, causing the rest of the elements to shift back.
LinkedList<Integer> deque = new LinkedList<>();
deque.addFirst(3);
deque.addFirst(2);
  1. Pop (removeFirst): The removeFirst method removes and retrieves the element from the front of the deque. It throws an exception if the deque is empty.
int first = deque.removeFirst(); // removes the first element
  1. Offer (addLast): The addLast method appends an element to the end of the deque. It adds the specified element as the last element of the list.
deque.addLast(4);
deque.addLast(5);
  1. Poll (removeLast): The removeLast method removes and retrieves the element from the end of the deque. It returns null if the deque is empty.
Integer last = deque.removeLast(); // removes the last element
  1. Peek (getFirst, getLast): The getFirst and getLast methods retrieve the first and last elements of the deque, respectively, without removing them. They are helpful in checking the elements at both ends before performing any further operations.
int firstElement = deque.getFirst(); // retrieves the first element
int lastElement = deque.getLast();   // retrieves the last element

LinkedList's implementation of queue and deque operations provides an efficient and effective way to handle data in a First-In-First-Out or Double Ended Queue manner. Whether you need a simple queue or a deque with more flexibility, LinkedList has got you covered.

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