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import LinkedList from '../linked-list/LinkedList';

export default class Queue {

constructor() {

// We're going to implement Queue based on LinkedList since the two

// structures are quite similar. Namely, they both operate mostly on

// the elements at the beginning and the end. Compare enqueue/dequeue

// operations of Queue with append/deleteHead operations of LinkedList.

this.linkedList = new LinkedList();

}

/**

* @return {boolean}

*/

isEmpty() {

return !this.linkedList.head;

}

/**

* Read the element at the front of the queue without removing it.

* @return {*}

*/

peek() {

if (!this.linkedList.head) {

return null;

}

return this.linkedList.head.value;

}

/**

* Add a new element to the end of the queue (the tail of the linked list).

* This element will be processed after all elements ahead of it.

* @param {*} value

*/

enqueue(value) {

this.linkedList.append(value);

}

/**

* Remove the element at the front of the queue (the head of the linked list).

* If the queue is empty, return null.

* @return {*}

*/

dequeue() {

const removedHead = this.linkedList.deleteHead();

return removedHead ? removedHead.value : null;

}

/**

* @param [callback]

* @return {string}

*/

toString(callback) {

// Return string representation of the queue's linked list.

return this.linkedList.toString(callback);

}

}